To obtain evidence of the dose-rate effect on induction of micronuclei in early spermatids, we observed frequencies in wild-type p53(+/+), heterozygous p53(+/-) and null p53(-/-) mice 14 days after gamma rays irradiation at a high (1,020 mGy/min) or a low (1.2 mGy/min) dose-rate. A dose- and dose-rate-related increase in micronuclei was seen in early spermatids with no difference between the different p53 status. These data were found to be best fitted by a linear-quadratic dose-response model at a high dose-rate, and by a linear dose-response model at a low dose-rate. The yields at 1.2 mGy/min were significantly lower than those at 1,020 mGy/min in the same manner, independent of p53 status. Testis weight declined significantly after 3 Gy irradiation, but did not depend on dose-rates. In our other studies, we observed the complete elimination both of malformation in fetuses and CD3- 4+ mutant T-lymphocytes in p53(+/+) mice, but not in p53(-/-) mice after irradiation. This indicates that concerted DNA repair and p53-dependent apoptosis are likely to completely eliminate mutagenic damage from the irradiated tissues at low doses or dose-rates in teratogenesis and lymphocytes. In the germ cell, however, irradiation at 1.2 mGy/min was mutagenic, independent of p53 status.