Various endonucleases (AluI, DNase I, benzon nuclease) and bleomycin induce sister-chromatid exchanges (SCE) in Chinese hamster ovary (CHO) cells. The frequencies of SCE are elevated in cells with chromosome-type aberrations, only slightly elevated in cells with chromatid exchanges, and in the control range in cells without chromosomal aberrations. These data indicate that SCE are produced when DNA breaks induced in G1 are either not repaired or misrepaired.