Female CB mice were chronically gamma-irradiated in utero during either of two periods, the 10th to 14th days or the 14th to 18th days of gestation. The doses administered were 34 rad/genaration in the earlier group and 160 rad/generation in the latter with dose rates of 0.3 rad/h and 1.7 rad/h, respectively. The doses were given through 9 generations. The effect of the irradiation was expressed as an increased frequency in the rate of recessive lethal equivalents by just above 4%. This corresponds to a mutation rate of 0.00015 mutation/rad/genome during the 10th to 14th gestational days and 0.00003 mutation/rad/genome in the 14th to 18th day group. As in earlier investigations, neither dominant mutations nor dominance effects of induced recessive lethal equivalents were found.