The mutagenic effects of 1-nitropyrene, and 1,3- and 1,8-dinitropyrenes on Chinese hamsters V79 cells in the presence or absence of X-irradiated Syrian hamster embryo cells were examined. Without Syrian hamster embryo cells, 1,3-dinitropyrene hafd weak mutagenic activity and 1,8-dinitropyrene had strong, dose-related mutagenic activity. With Syrian hamster embryo cells, the mutagenicities of both 1,3- and 1,8-dinitropyrenes were appreciably increased. 1-Nitropyrene did not induce ouabain-resistant mutants at concentrations of up to 10 .mu.g/ml either with or without Syrian hamster embryo cells.