Asbestos fibers were tested for possible mutagenic activity using the Fluorescent Plus Giemsa (FPG) sister chromatid exchange (SCE) technique. Amosite, crocidolite, and chrysotile fibers were added to cell cultures at final concentrations of 10 and 100 micrograms/ml. Chrysotile completely inhibited cell growth at both concentrations; cells exposed to amosite and crocidolite proliferated but only at the lower concentration. Crocidolite significantly elevated the SCE rate and larger (greater than 5 mu) chromosomes were most sensitive. Amosite appeared to have a lesser effect on SCE frequency. Asbestos fibers are capable of disturbing cellular processes associated with chromosomal stability and effects vary with the asbestos type.