The capacity of asbestos to mediate transfection was tested in a rapid and relatively simple system: picornavirus RNAs and mammalian cells in vitro. Thirteen asbestos samples, including amosite, anthophyllite, chrysotile, and crocidolite, 4 picornaviruses (poliovirus 1 and 2, echovirus 7, and encephalomyocarditis virus), and 4 cell lines (CLI, chimpanzee liver; KB, human carcinoma; eta, monkey kidney; NIH 3T3, mouse embryo) were tested. The results showed that all asbestos samples mediated transfection and that all cell lines were transfectible by viral RNA with asbestos. Transfection was much greater with asbestos added to the viral RNA inoculum than to the cells before or after the RNA. Transfection was directly proportional to asbestos concentration. Initiation of transfection events was rapid, with half becoming irreversible by washing 2 min postinoculation. DNA in the inoculum strongly interfered with asbestos-mediated transfection by the RNA but was ineffective when added, with or without asbestos, to the cells before or after the inoculum. Asbestos compared with six classical "insoluble" facilitators (bentonite, calcium phosphate, chromic oxide, ferric oxide, kaolin, talc) was of intermediate rank in transfection mediation. It is hypothesized that the prominence of asbestos in carcinogenesis is due to a combination of properties, including transfection mediation as well as chromosome mutagenicity, fiber dimensions, biological durability, hydrocarbon transport, and prevalence.