The nature, degree, and kinetics of unscheduled thymidine incorporation previously shown to occur in 90% of irradiated lymphocytes was studied. The incorporation was severely depressed in the presence of 10-4 M acriflavine and by low temperature, but was unaffected by 10-3 M hydroxyurea or caffeine. Over a dose range of 25 to$400\ {\rm ergs}/{\rm mm}^{2}$, the uptake of thymidine was increased by a factor of only 1.6, although the survival of lymphocytes, measured 5 days after irradiation, decreased by almost two orders of magnitude. (The survival curve suggests that 90% of the lymphocytes have a$D_{0}\ \text{of}\ 35\ {\rm ergs}/{\rm mm}^{2}$ and 10% have a D0 of$250\ {\rm ergs}/{\rm mm}^{2}$.) After exposure to$25\ {\rm ergs}/{\rm mm}^{2}$, over 70% of the cells survived for 5 days in culture; moreover, cells which had been stimulated by this dose to incorporate thymidine transformed and divided after exposure to phytohemaglutinin. The final uptake of thymidine was significantly greater when a total dose of$75\ {\rm ergs}/{\rm mm}^{2}$ was fractionated into three doses of$25\ {\rm ergs}/{\rm mm}^{2}$ given at six hourly intervals than when it was given as a single dose. The degree of thymidine incorporation and the fraction of leukemic cells labeled were not significantly different from those in normal lymphocytes.

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