We have studied the synergistic action (enhancement and inhibition) of individual aromatic hydrocarbons containing from 1 to 7 rings, on the mutagenicity of benzo[a]pyrene (B[a]P). Several non-mutagenic hydrocarbons (2 and 3 unsubstituted rings) enhanced the mutagenicity of B[a]P. On the other hand, most mutagenic polycyclic aromatic hydrocarbons (PAH) produced a large decrease and sometimes a complete abolition of the mutagenicity of B[a]P. In addition, these higher PAH also exhibited an activating effect of B[a]P mutagenicity, at low doses. The higher the number of cycles of the PAH, the lower the amount of PAH necessary to enhance and (or) to inhibit the mutagenicity of B[a]P. Metabolic activation of B[a]P was evidently involved in synergistic phenomena because no effect was observed on B[a]P-4,5-oxide, a direct-acting metabolite of B[a]P.