Trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (BP-7,8-diol) and the anti-isomer of trans-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) were found to be activated by microsomes isolated from 3-methylcholanthrene (MC)-treated rats to reactive intermediates that bound covalently to microsomal proteins. The extent of binding was markedly reduced by the presence of reduced glutathione (GSH) or cysteine. Fluorescence spectroscopic studies on the products derived from BP-7,8-diol and BPDE after microsomal activation in presence of GSH or cysteine revealed the formation of a common reactive intermediate with unique fluorescence properties. The involvement of cytochrome P-448 in the activation of BP-7,8-diol and BPDE to protein-binding products was inferred by the requirement for NADPH and almost complete inhibition by alpha-naphthoflavone. Furthermore, microsomes from MC-treated rats could be replaced by a reconstituted system containing purified cytochrome P-448, NADPH-cytochrome reductase and co-factors. The conjugation of the reactive intermediates from BP-7,8-diol and BPDE with GSH or cysteine did not require the presence of either microsomes or cytosol, thus indicating a non-catalytic reaction. These results emphasize the importance of cellular nucleophiles such as GSH and cysteine in the deactivation of reactive benzo[a]pyrene (BP) intermediates and also provides evidence for the further activation of the ultimate carcinogen BPDE to more reactive electrophiles and may thus have relevance concerning the regulation of BP-induced carcinogenesis.