CBA X C57BL (F1) mice of both sexes and C57BL were placed within 10 km of the reactor for 14, 22 or 34 days. The gonadal absorbed radiation ranged from 0.5 to 1.5 Gy. The frequencies of dominant lethal mutations, abnormal sperm heads and reciprocal translocations increased linearly with increasing radiation dose. Among 49 exposed as embryos, 1 was heterozygous for a reciprocal translocation.