Adult Balb/c Kh male mice were irradiated (pelvic region, 250 kVcp X-rays, 60 rad per min) and three months later were mated to untreated C57BL/6 Kh females. Their B6C F1 progeny were screened for mutations at the Class II histocompatibility loci, i.e. those that carry similar alleles in the parental lines and are therefore homozygous in the F1 progeny. The treatment groups were: single doses of 0, 350, 500, 650 and 800 rad; split doses 1 day apart, totalling 500, 650 and 800 rad; split doses averaging 52 days apart, totalling 650 and 800 rad. Thirty-six mutants were identified in 13,614 progeny. Twelve of them occurred in five clusters of two or three, presumably owing to five gonadal mosaics among 940 parents. Irradiation did not increase the spermatogonial mutation rate. The only effect of exposure appeared to be a decrease in the mutation rate of the 1-day split dose-groups compared to those with the same total doses in a single exposure or in two fractions, 52 days apart.