Several nitroarenes derived from benzo[a]pyrene and perylene and the parent hydrocarbons have been assayed for mutagenicity in theSalmonellamicrosome test and for affinity for the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-receptor protein in rat liver cytosol. 1- and 3-nitrobenzo[a]pyrene are mutagenic in the absence of S9 and have a response pattern in severalSalmonellastrains resembling that shown by 1-nitropyrene. 6-Nitrobenzo[a]pyrene, 3-nitroperylene as well as the parent unsubstituted hydrocarbons require S9 for activation. The 3,6- and 3,7-dinitroperylenes and a mixture of 3,9-/3,10-dinitroperylene are all mutagenic in the absence of S9. The response pattern of 3,9-/3,10-dinitroperylene resembles that shown by 1,6- and 1,8-dinitropyrenes. The nitro derivatives which are active in the absence of S9 are all inactivated by the addition of an incomplete S9 lacking NADP but activity is regained in the presence of the complete S9 for 1-and 3-nitrobenzo[a]pyrene and 3,6-dinitroperylene, showing that these compounds are also activated by S9 enzymes. Benzo[a]pyrene, 1- and 3-nitrobenzo[a]pyrene, 3-nitroperylene and the mixture of 3,9-/3,10-dinitroperylene have a high affinity for the TCDD-receptor protein whereas the affinity is low or below the detection level for the other compounds. These results are in good agreement with known structural requirements for receptor binding.