Abstract: Male mice were treated with cyclophosphamide (CY) and then serially mated to superovulated female mice over the whole spermatogenic cycle. Chromosome preparations were made from fertilized ova obtained from the mating. No chromosomally abnormal ova were found in mating using untreated animals, but mating involving CY - treated males induced 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 in mice treated with CY at the same stage were not significantly inhibited, but the fertilizing ability of sperm in vitro was decreased, as well as the embryo development was remarkably delayed as compared to those in controls (P<0. 01).

Key words: "Male germ cell, Chromosome, Cyclophosphamide, Fertilization in vitro , Embryo development, Mouse "