The dose-response curve for reciprocal translocations induced by X-rays in spermatogonial stem cells, and observed in primary spermatocytes of mice, is "hump-shaped", with a maximum yield at about 600 R. To test the hypothesis that the decrease in yield with increasing dose above 600 R is a consequence of the different sensitivities of cells in different stages of the cell cycle to both cell killing and chromosome aberration induction, several fractionation experiments were carried out. A total dose of 2800 R was given in repeated doses of 400 R, separated by 8-week intervals. The yield of translocations is that expected for additivity; for example, the yield at 1600 R is approximately equal to that for four separate 400-R doses. When a total dose (500 R) which gives a translocation yield on the ascending part of the dose-response curve is given as two equal fractions separated by intervals of 30, 90, or 150 min, the translocation yield decreases with increasing interval. However, when a total dose (1000 R) which would give a translocation yield on the descending part of the dose-response curve is given in two equal fractions separated by intervals of from 30 min to 6 weeks, the response is different; the translocation yield increases with intervals up to 18 h, then decreases with intervals up to 4 weeks, and finally increases again to a yield equal to additivity with an interval of 6 weeks. These changes in translocation yield with changes in interval between the two doses are explained in terms of the differential sensitivity of cells to killing and aberration induction in the different phases of the cell cycle, and by assuming that the cells surviving the first dose and repopulating the testis have different cycle characteristics from normal cells.