Third generation isotransplants of a C3 H mouse mammary carcinoma were treated with either a single dose or with two doses of <tex-math>${}^{134}{\rm Cs}$</tex-math> gamma rays. In the two dose studies, the first dose (D1) of 620 rads was given under ambient conditions. At intervals of 24, 48, or 96 hours after D1, the second dose (D2) was administered under one of three conditions: hypoxia, ambient, or <tex-math>${\rm O}_{2}30$</tex-math> psi. The range of doses administered at D2 was chosen to allow estimates of <tex-math>${\rm TCD}_{50}/120$</tex-math> days to be made for each time interval and each condition of irradiation. For each condition of irradiation at <tex-math>$D_{2},\ {\rm TCD}_{50}$</tex-math> was observed to be a minimum for a time interval of 48 hours between D1 and D2. This effect was interpreted as an indication of a partial synchronization of tumor cells surviving D1 rather than an improvement in oxygenation. Partial synchronization may have resulted in a grouping of tumor cells into relatively sensitive stages of the cell replication cycle. At 96 hours after D1 there was an apparent increase in the proportion of aerobic tumor cells probably due to a relatively more rapid increase in cell number in the aerobic than in the hypoxic compartment, rather than to reoxygenation per se.

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