Studies were conducted to determine the effects of a single, split and fractionated doses (separated by 1-h intervals) of 100 rad of X-irradiation on the morphological transformation of BALB/c 3T3 clone A31-11 mouse fibroblasts grown in media containing calf serum. Both spontaneous and radiation-induced transformation levels were lower for these cells grown in the cell-serum containing media than previously reported for these cells grown in fetal calf serum containing media. In the studies reported here, cells were irradiated either as density-inhibited plateau phase cultures or as low density cultures at 10-14 h after being reseeded from confluent dishes. We observed that a 4-fraction 100-rad dose resulted in a reduced yield of transformants compared to a single dose of 100 rad when plateau phase cultures were utilized for the radiation exposures, but not in low density cultures in which the cells were allowed to proliferate during the radiation exposures, these results suggest that the growth phase of the cells can play a major role in determining the yield of transformants induced by fractionated doses of radiation. It is noteworthy that, for the other data obtained in these studies, in none of 12 different experimental points (involving 5 separate experiments) did a fractionated dose protocol result in a reduced yield of transformants when compared to a single dose protocol.