The coat and eye colour mutant beige (bg) leads to the production of distinctive retinal melanocytes with abnormally large pigment granules. Heterozygotes for bg were given 2 Gy acute X-irradiation at various times between day 11.5 of fetal life and 3 days after birth, at which age whole mounts were prepared of the retinal pigmented epithelium (RPE). These were scanned for the presence of mutant retinal melanocytes with large granules, either as single cells or as clones. The earlier the fetal irradiation, the greater was the effect on RPE area at 3 days post-partum (p.p.), which fell to about half normal with the 11.5-day fetal exposures. However, the ultimate size of the retinal melanocytes seemed little affected by the irradiation, although their normal size increased .apprx. 3-fold between 12.5 days post-coitum (p.c.) and 3 days p.p. Mean numbers of mutant melanocytes per eye were markedly higher than in +/bg controls at all irradiation ages other than 3 days p.p.; when allowance was made for final sizes of irradiated RPE''s mutation frequencies fell steadily from 30.0 .times. 10-5 at 11.5 days p.c. to 1.0 .times. 10-5 at 3 days p.p., with 0.8 .times. 10-5 in +/bg and 0.1 .times. 10-5 in +/+ controls. The doubling dose of 0.18 Gy at 16.5 days p.c. was similar to that found at 17.5 days p.c. in a previous somatic mutation experiment in which follicular melanocytes were scanned for mutations at different (d and ln) loci. In the present experiment, numbers of mutant melanocytes per clone tended to decrease with later irradiation, but not to the extent expected from the greatly increased numbers of cells at risk over the same period. It was concluded that the bg test system showed great promise for the study of in vivo mutagenesis in somatic cells, especially as the fetal RPE is pigmented from 11.5 days p.c. in the mouse.