Chinese hamster lung cells cultured in the presence of amosite, chrysotile or crocidolite displayed inhibition of cell growth. Phagocytosis of asbestos by cells was observed. Amosite and chrysotile appeared to be twice as toxic as crocidolite based on an equal weight/volume basis. Induction of 6-thioguanine resistance was studied. After treatment at 10 microgram/cm2 for 24 h, and with a 48-h expression time, numerically the cells did not change significantly from 6-thioguanine-sensitive to 6-thioguanine-resistant for each pair of control and experimental mutation frequencies. When all data points were considered as a whole, use of Student's t-test on paired samples showed that all 3 types of asbestos were mutagenic in Chinese hamster lung cells. It was reported that benzo[a]anthracene was an organic contaminant of abestos. Used as a positive control compound, 7,12-dimethylbenz[a]anthracene was metabolized by rat-liver microsomes and was found mutagenic in Chinese hamster lung cells. Microsome activation did not increase the mutagenic potential of asbestos. These findings point to the weak mutagenic potential of asbestos. Chinese hamster lung cells phagocytize particles and are ideal materials to study mutagenesis of environmental agents that exist in a particulate form.