NMRI mice were treated with single doses of cyclophosphamide (Cytoxan) and spermatogonia were analysed for chromosome aberrations at various time intervals after treatment. The maxima of aberrations were found 24 hrs p.i. Chromatid type aberrations were observed exclusively. About half of the aberrations consisted of chromatid interchanges, 92% of which exchanging short arm fragments close to the centromeric region. The lack of meiotic multivalents in diakinesis-metaphase I after treatment of spermatogonia stem cells with cyclophosphamide in the study of Leonard and Linden (1972) is discussed.