Erythrocytes (RBC) from mice given phenylhydrazine are protective when injected into syngenic mice before X irradiation. The <tex-math>${\rm LD}_{50/30}$</tex-math> was increased about 90 R when these RBC (PhRBC) were given iv 1 day before exposure. PhRBC were not protective when given 4 hr after irradiation. Radioprotection by PhRBC did not appear to result from either an increase in the radioresistance (D 0 ) of hemopoietic stem cells or an increase in number of stem cells at risk at the time of irradiation. Using blood clearance and liver accumulation of labeled sheep RBC as a measure of reticuloendothelial activity, we found no evidence that a stimulated reticuloendothelial system was responsible for radioprotection by PhRBC. Some evidence suggested that repair of hemopoietic stem cells was enhanced in mice injected with PhRBC before X irradiation.

Injection of S-2-[3-aminopropylamino]ethylphosphorothioic acid (WR-2721) into BALB/c or ${\rm BC}3{\rm F}_{1}$ mice induces a dramatic vasodilation, which is most pronounced in the spleen. This response is a component of the drug's toxic mechanism, as evidenced by the fact that splenectomy increases the tolerance of the mice to the toxic effects of the drug. Further, removal of the spleen is associated with a reduced effectiveness of the drug in terms of radioprotection, which is similar to the effects of reduced oxygen tension. Thus, at least part of the superiority of WR-2721 as a radioprotectant is due to its ability to reduce peripheral oxygen tension. Analysis of a number of phosphorothioates has demonstrated that they fall into two general categories, those that can induce the pharmacologic response and those that cannot. Each class is described by a single curve of protection versus amount of sulfur injected. The pharmacologically active drugs produce a two-component curve-an initial rapid rise followed by a less rapid increase. The second component parallels the linear curve that characterizes the entire dose range for the drugs that are not pharmacologically active. We conclude that the radioprotective effectiveness of phosphorothioates in general is determined by two factors, the amount of sulfur that can be injected and the presence or absence of the pharmacologic reaction.

Phenylhydrazine is a radioprotector when injected into mice 6 to 11 days before exposure to x-rays. To better define the basis for radioprotection, the following studies were made: effects of phenylhydrazine on <tex-math>${\rm LD}_{50(30)}$</tex-math>, hemopoietic regeneration in irradiated mice, hemopoiesis of unirradiated mice, hemopoietic cell radiosensitivity, and life-span of protected mice. Injection of 3.0 mg of the drug a week before irradiation increased the <tex-math>${\rm LD}_{50(30)}$</tex-math> by 140 R, which is equivalent to a dose reduction factor of about 1.2. Phenylhydrazine-treated mice survive because of hematologic recovery that becomes manifest 2 weeks after irradiation. Some of the results are consistent with the hypothesis that radioprotection by phenylhydrazine is the result of increased numbers of hemopoietic stem cells in the spleen at the time of irradiation. However, some of the colony-forming unit and splenectomy data indicate that this increase may not be the sole basis for protection, although it is clear that the spleen is involved. Protection by phenylhydrazine does not appear to be related to an increase in radioresistance of hemopoietic stem cells (colony-forming units) or to hypoxia resulting from anemia or from methemoglobin formation.

The importance of the spleen to recovery of lethally x-irradiated mice injected with syngeneic bone marrow was evaluated. When splenectomy was performed 2 days before irradiation, injection of marrow resulted in 30-day survival, which was the same as that of intact mice. However, when splenectomy was performed 2 or 3 days after exposure and marrow injection, survival was decreased, probably because the fraction of marrow committed to the spleen was removed. Results of ferrokinetic studies ( 59 Fe) showed that early regenerative erythropoiesis in lethally irradiated mice injected with marrow is reduced in animals splenectomized before irradiation, but only when the marrow cell dose is limiting. Even at a limiting cell dose, erythropoiesis is eventually restored to that of intact mice. Thus the spleen appears to contribute to early restitution of a hemopoietic compartment least critical to survival of the animal, and whatever contribution the spleen makes to survival is compensated by other tissues when the spleen is absent. Interpretation of splenic contribution to hemopoietic regeneration and survival is complicated by the fact that splenectomy performed 2 days before irradiation increases the <tex-math>${\rm LD}_{50/30}$</tex-math> about 65 R. This kind of protective effect, however, is also manifested by uninephrectomy but not by partial hepatectomy or uniorchidectomy. Our data shed no light on the basis for this kind of radioprotection.

The effects of very small doses of syngeneic bone marrow cells on 30- and 90-day survival of x-irradiated female (C57BL/Cum ♀ × C3H/Anf ♂) F 1 mice were studied, principally in the <tex-math>${\rm LD}_{10}-{\rm LD}_{100}$</tex-math> range. As few as 0.01× 10 6 cells improved survival of mice exposed to an <tex-math>${\rm LD}_{100}$</tex-math> or less, and a smaller cell dose was required to obtain good survival at mid-lethal exposures than at or above the <tex-math>${\rm LD}_{100}$</tex-math>. The data, together with previous results, indicate a triphasic response of the <tex-math>${\rm LD}_{50(30)}$</tex-math> to the dose of syngeneic marrow cells. The <tex-math>${\rm LD}_{50(90)}$</tex-math> and <tex-math>${\rm LD}_{50(30)}$</tex-math> values are essentially the same, except for mice exposed to 1100, 1150, and 1200 R. In these groups 90-day survival percentages were about one-half those scored at 30 days. It was also found that slopes of survival curves for mice treated with marrow were about one-half those for x-ray control mice, implying that injected cells are more efficient at repopulation than host cells which survive exposures approaching <tex-math>${\rm LD}_{100}$</tex-math>.

Radiation response of mouse bone marrow cells in terms of their erythropoietic activity was determined as a function of the following in vitro environmental conditions: (1) temperature during and after (up to 6 hours) X-irradiation and (2) temperature during the incubation period between two X-ray exposures. Results indicate that (1) radiosensitivity is not a function of the temperature (2°C, 25°C, 37°C) during irradiation; (2) in the range 2° to 30°C, incubation temperature during the 6-hour period after irradiation (100 R) has no detectable influence on radiation response of cells; (3) in the 30° to 40°C range, injury to irradiated cells is directly related to incubation temperature; and (4) the degree to which cell survival is enhanced by fractionation of the exposure is directly related to the temperature at which cells are incubated between the two exposures.