Styrene oxide is multagenic, without metabolic activation, to S. typhimurium strains TA 1535 and TA 100, which have been devised to detect mutagens causing base-pair substitutions. Styrene seems to be mutagenic toward the same strains, but only after metabolic activation. The toxicity of both styrene and styrene oxide make the construction of reliable dose-response curves rather difficult. Diethylmaleate and 3,3,3-trichloropropene oxide enhanced the mutagenicity of styrene oxide in the presence of homogenate; this result indicates the participation of epoxide hydratase and glutathione S-oxide transferase in the metabolism of styrene oxide. These two chemicals did not influence the mutagenic activity of styrene. Styrene glycol and 4-tert-butyl-brenzcatechin were not mutagenic to any of the strains studied. Results show that further, more detailed experimental and, possibly, epidemiologic studies are warranted.