Three forms of Union Internationale Contre le Cancer (UICC) asbestos, amosite, crocidolite, and chrysotile, were assayed for their cytotoxicity (inhibition of colony formation) in cell culture. Using embryonic human intestine-derived (I-407) and adult rat liver-derived (ARL-6) epithelial cells, the order of cytotoxicity was chrysotile>amosite>crocidolite. All three asbestos types were more toxic to I-407 than to ARL-6 cells. Chrysotile, amosite, and crocidolite were also tested for inhibition of colony formation in cultures of mouse colon-derived epithelial-like (MCE-1) cells; these cells were more resistant than the I-407 cells to all three fiber types; although similar to the ARL-6 cells in sensitivity to chrysotile and amosite, the MCE-1 cells were more sensitive to crocidolite than the ARL-6 cells. Leaching of the three asbestos forms for 3 days in sterile deionized water did not appreciably affect the cytotoxicity of I-407 or ARL-6 cells. Leaching in hydrochloric acid, however, greatly decreased the cytotoxicity of chrysotile on these cells and particularly in cultures of ARL-6 cells slightly increased the cytotoxicity of amosite and crocidolite. Leaching in deionized water altered the composition of the asbestos as revealed by release of substantial levels of Mg2+ and Ca2+ into the leaching fluid. Greater titers of these ions were released during leaching in hydrochloric acid.