The effects of a radiomimetic chemical, bleomycin (BLM), on meiotic chromosomes was evaluated in mice. Chromosome aberrations were analyzed at meiotic metaphase I, and damage to the synaptonemal complex was analyzed in meiotic prophase cells. In the metaphase aberration studies, an ip. injection of 80 mg/kg BLM, timed to precede or coincide with ultimate S-phase, led to a significant increase in structural damage (p<0.01) in cells reaching metaphase I 12 d after treatment. However, no increases in clastogenic effects were observed at metaphase I after treatment of cells during various stages of prophase. SC analyses in pachytene cells following an ip. or it. injection at S-phase revealed various forms of synaptic errors and structural anomalies, including qualitative changes similar to those observed following irradiation. Ip. doses ranging from 25-100 mg/kg, and it. doses as low as 0.5 mg/kg, caused roughly sixfold increases over control levels in the number of damaged cells. It was concluded that BLM is clastogenic for meiotic chromosomes; however, it does not reveal the strong S-independedt clastogenic activity at meiosis that is characteristic of its activity at mitosis.