Male mice were treated with Cyclophosphamide(CY) and then serially mated to superovulated female, over the whole of the spermatogenic cycle. Chromosome preparations were made from fertilised eggs obtained from the matings. No chromosomally abnormal eggs were found in matings using untreated animals but mating involving CY-treated males produced a variety of abnormalities. The most sensitive stage in the spermatogenic cycle was 8 days after treatment, corresponding to the testicular sperm stage of spermatogenesis. The spermatozoa concentration and activity, as well as fertilizing ability of sperm in vitro in mouse treated with CY at the same stage was not significantly inhibited, but the embryonic development was remarkably delayed, compared to controls (<0.01).