Asbestos, a proven carcinogen, is reported to have no genotoxic effects. We hypothesized, however, in light of its clastogenic effects that one mechanism by which asbestos induces cell transformation and tumorigenesis involves the induction of DNA strand scission. Cultured rat embryo cells were exposed to low concentrations of International Union Against Cancer crocidolite and examined at intervals ranging from 2 to 48 h. The induction of DNA strand breaks was examined using the technique of nick translation followed by autoradiography or scintillation counting. Our results indicate that cells exposed to crocidolite have a higher incidence of DNA breaks and that this effect becomes apparent within 2-6 hours of exposure. Ball-milled crocidolite as well as riebeckite have a significantly lower effect while glass fibers induce a more pronounced DNA strand damage. These observations support the role fiber length plays in carcinogenesis and suggest that the classification of asbestos as a nongenotoxic carcinogen be reconsidered.