Male C3H mice were exposed to 100 W m-2 of 2.45 GHz continuous-wave microwave radiation for 6 h per day for a total of 120 h over an 8-week period. The exposure level was chosen so that the specific energy absorption rate (SAR) would be approximately equal to the level of 4 W kg-1 which is considered by a number of organizations to be a threshold for adverse biological effects. At the end of the treatment period the mice were mated with a different group of (C3H x 101) F1 hybrid females each week for the following 8 weeks. There was no significant reduction in pregnancy rate, preimplantation survival or postimplantation survival in the exposed group compared to sham-exposed controls. At the end of the mating period a cytogenetic analysis was carried out of meiotic chromosome preparations of testicular tissue, thus sampling cells that were stem cell spermatogonia during the treatment regime. The results showed no difference in the frequency of reciprocal translocations between the sham and treated groups, or in the frequency of cells with autosome or sex chromosome univalents. Low levels of fragments and exchanges were found in both groups. It is concluded that there is no evidence in this experiment to show that chronic exposure of male mice to 2.45 GHz microwave radiation induces a mutagenic response in male germ cells. This conclusion is in agreement with the observations of Berman et al. (1980), who reported a lack of male germ cell mutagenesis after repetitive or chronic exposure of rats to 2.45 GHz.