Investigation was made of the amount of dominant lethality found among offspring of male mice after irradiation of the spermatogonia with 550 R X-rays. A significantly greater rate of death was observed in the X-ray group when compared with the corresponding controls. The effects were constant and continued over a long period of time. The effect was also evident in outcrosses to another strain. No evidence of fluctuations in the results which might indicate intra-tissue selection or clustering was noted. Tests of F 1 sons of the irradiated and control groups presented no differences between the groups. There were no indications of chromosomal effects leading to semi-sterility, or dominant lethal genes with incomplete penetrance producing higher death rates. A higher rate of intra-uterine death was observed among F 1 females of the X-ray group when compared to the control.