The ability of 1,6-dinitropyrene and 1,8-dinitropyrene to induce chromosome aberrations has been examined in a rat epithelial cell line (RL4). Both compounds were found to be potent clastogenic agents in these cells, inducing predominantly chromatid type abberations. The number of aberrant metaphases was dose-related up to 1.25 micrograms/ml, after which the response showed a plateau at a frequency of around 60%. The number of chromatid gaps per 100 cells also increased linearly with concentration from 3.5 in the control cultures up to 117 with 1,6-DNP (1.25 micrograms/ml) and 98 with 1,8-DNP (2.5 micrograms/ml), indicating that the induction of chromatid gaps was a valid and sensitive measure of chromosome damage in this cell line. The fact that a positive result as obtained in RL4 cells implies that these cells contain enzyme systems which are capable of converting dinitropyrenes to a mutagenic form.