Female C3H mice were fed a standard pellet diet (containing 0.2 ppm Se and 30 ppm vit. E) or the same diet supplemented with 0.8 ppm (low dose) or 3.4 ppm (high dose) of selenomethionine for 10 weeks. After mating with males receiving the standard diet the mice were subjected, on the 9th day of pregnancy, to whole body roentgen irradiation of 1.75 Gy. On day 18 of gestation the frequency of resorptions, mortality and the incidence of fetal malformations were studied. Supplementation with Se-methionine resulted in a significant but dose-independent decrease (p less than 0.005) of the number of malformed fetuses from 62 per cent in the irradiated controls to 47 per cent in the low Se-group and high Se-group, respectively. In addition, the number of total malformations as well as fetal resorptions were significantly decreased in a dose-independent manner in the supplemented groups. The decrease in fetal malformations occurred proportionally for all the major malformations observed, i.e. short or kinked tail, rib and vertebral malformations, coloboma and deformation of retina and iris. Glutathione peroxidase activity in whole blood of Se-methionine fed mice was significantly increased. Thus, Se-rich diet may result in scavenaging of radiation-induced hydroperoxides.