Ten polycyclic aromatic hydrocarbons (PAHs), viz. anthracene pyrene, chrysene, perylene, fluoranthene, benzo[a]pyrene, benzo[a]pyrene, benz[a]anthracene, benzo[ghi]perylene, benzo[k]fluoranthene, have been nitrated using concentrated nitric acid and the crude nitrated mixture examined for biological activity. All the nitro PAHs examined were mutagenic to Salmonella typhimurium in the absence of a rat liver preparation. Addition of Aroclor-1254 induced liver had little effect on mutagenicity. Mutagenic potency differed for the various nitrated mixtures with nitrated pyrene and nitrated fluoranthene the most potent and nitrated anthracene the least potent. Both frame-shift and base-substitution mutations were induced by the nitrated PAHs. The nitrated PAHs were also able to induce DNA repair synthesis in cultured HeLa cells in the absence of liver, indicating that these cells have the necessary enzymes to activate nitro PAHs. Potency again varied from compound to compound with nitrated pyrene appearing to be the most active. Isolation of individual components from the crude nitrated mixtures has not been carried out in this study. In view of the possible wide-spread distribution of nitrated PAHs in the environment further work is required to assess the carcinogenic potency of these compounds which possibly pose a risk to man.