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 nor do they appear to cause DNA strand breaks in either rodent or human cells. 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 genetoxicity of fibers. It is reasonable to expect, therefore, that these materials may be oncogenic by virture of mechanisms other than as tumor initiators.