Abstract Mechanisms of co-carcinogenicity of particulates, such as iron oxide and asbestos, and benzo[ a]pyrene (B[ a]P) are not completely understood. Particulates dramatically alter rates of uptake of B[ a]P into membranes, a factor which could account for co-carcinogenicity. However, B[ a]P must be activated to reactive forms to be carcinogenic and mutagenic so alterations in metabolism of B[ a]P by particulates also could result in co-carcinogenesis. To elucidate mechanisms of particulate-B[ a]P co-carcinogenesis, we have correlated rates of uptake of B[ a]P into microsomes with metabolism of B[ a]P and with mutagenicity of B[ a]P in the Ames test. In general, aryl hydrocarbon hydroxylase (AHH) activity paralleled rates of uptake of B[ a]P, though some inhibition of AHH activity by particulates which was not attributable to availability of B[ a]P was evident. This inhibition was studied further by assaying separately mixed function oxidase and epoxide hydrase activities in the presence of particulates. Both chrysotile and iron oxide inhibited O-deethylation of 7-ethoxyresorufin and hydration of B[ a]P-4,5-oxide. To determine effects of this inhibition on activation of B[ a]P to reactive forms, we studied profiles of metabolites of B[ a]P and mutagenicity of B[ a]P. The only alteration in profiles of B[ a]P metabolites produced by particulates was that due to effects on rates of uptake. Similarly, mutagenicity of B[ a]P was positively correlated with rates of uptake into microsomes. We conclude that the predominant effects of chrysotile and iron oxide are in altering rates of uptake of particle-adsorbed B[ a]P. Changes in uptake rates then result in alterations of B[ a]P metabolism and mutagenicity.