Asbestos fibers are highly cytotoxic to cultured mammalian cells and produce chromosomal aberrations in several rodent cell types. There is some uncertainty in the literature as to whether these fibers are clastogenic to cultured human cells. Asbestos fibers do not produce either DNA damage or back mutations in prokaryotic assay systems, and they do not appear to cause DNA strand breaks in either rodent or human cels. The evidence that these fibers can produce either forward mutation or neoplastic transformation of mammalian cells is weak. Asbestos fibers are clearly oncogenic to humans and animals, but, except for clastogenic effects in rodent cells, there is little evidence for genotoxicity of fibers. It is reasonable to expect, therefore, that these materials may be oncogenic by virtue of mechanisms rather than as tumor initiators.