In order to assess the influence of p53 inactivation on radiation-induced developmental effects, male mice heterozygous for the wild-type p53 allele (mimicking the human Li-Fraumeni syndrome) were crossed with C57BL females, and their heterozygous p53+/- progeny were mated with each other to obtain p53+/-, p53-/- and p53+/+ embryos. Pregnant females were X-irradiated with 0.5 Gy on days 1 (pre-implantation period), 8 or 11 (organogenesis period) of gestation. Dissection of the pregnant females occurred on day 19 of gestation. The p53 genotype of the foetuses was determined by PCR from small pieces of soft tissues. Exencephaly was the only external malformation found in the control group. It affected essentially p53-/- female foetuses. A number of p53+/- and p53+/- control foetuses also showed dwarfism, or underdevelopment. In the group irradiated on day 1, the frequency of abnormal foetuses was, paradoxically, lower than that found in the control group. As in that group, exencephaly and dwarfism constituted the only anomalies that were found. Exencephaly affected only homozygous p53-/- females, while dwarfism concerned either p53-/- or p53+/- foetuses, with a majority of females. Irradiation on day 8 of gestation induced a significant increase in the frequency of abnormal foetuses, compared to the control group. Various malformations were observed in addition to exencephaly, including gastroschisis, polydactyly, cephalic oedema and cleft palate. All malformed foetuses were either homozygous p53-/- or heterozygous p53+/- while most affected foetuses were females, as was the case for dwarf individuals. Irradiation on day 11 did not cause an increase in the frequency of abnormal foetuses, in comparison with the controls. However, a large spectrum of external malformations was again noticed, as in the group irradiated on day 8. All affected foetuses were homozygous p53-/- and there were slightly more abnormal females than males (3 out of 5). No dwarfs were found in this group. Overall, these results confirm the importance of the p53 tumour-suppressor protein for normal embryonic development. They clearly show that homozygous p53-/- (or heterozygous p53+/- to a lesser extent) foetuses are more at risk for radiation-induction of external malformations during the organogenesis period, and that the risk of developing such malformations is much higher for females than for males. In contrast to results published very recently by others, we found that malformed foetuses resulting from an X-irradiation with a low-dose during the highly sensitive period of gastrulation are able to survive to birth.