CBA female mice were injected intraperitoneally with a dose of 20..mu..Ci of sulphur-35 on 15.5 day post conception. Another group of pregnant mice injected with normal saline was kept as control. The pregnant females were allowed to litter and the mothers were separated from their offsprings 4 weeks after littering. Eight weeks after treatment i.e. at the age of 22 to 24 weeks, the treated mothers were mated with control C/sub 3/H/He males. The vaginal plugs were checked every morning and those which mated were separated. The pregnant females were killed on the 14th day of gestation. The uterine contents were examined for live and dead embryos and the ovaries for corpora lutea. The pre- and post-implantation losses and total loss were calculated in the treated females and compared with those of controls. Embryonic death was significantly higher among treated animals. The results indicated that /sup 35/S can induced dominant lethal mutations in immature oocytes.