Mouse embryos in the pronuclear zygote stage were either irradiated in utero with a dose of 15 rad of fast neutrons or were sham-irradiated. Those animals that survived at least 30 days after birth were observed until their natural death. We investigated the percentage incidence and mean age at death for each of the principal neoplastic diseases seen on postmortem examination and also cumulative mortality distributions. No statistically significant differences were found between irradiated and sham-irradiated mice of the same sex.