The interaction between the sulfite anion and specific benzo[a]pyrene (B[a]P) derivatives produces a novel class of benzo[a]pyrene sulfonates. (+/-)-7,8,9-Trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene-10-sulfonate (B[a]PT-10-sulfonate) is formed in high yields in incubations containing (+/-)-7r,8t-dihydroxy-9t,10t-epoxy-7,8,9,10-tetrahydrobenzo[a]pyre ne (anti-BPDE) and sulfite, and sulfite strongly enhances the mutagenicity of the diolepoxide toward Salmonella typhimurium under those conditions. Although B[a]PT-10-sulfonate itself shows little direct mutagenicity over a 1-20 microM concentration range, this reactive bay-region intermediate does enhance the mutagenicity of anti-BPDE in strains TA98 and TA100 by up to 280%. No significant enhancement was seen when up to 20 microM B[a]PT-10-sulfonate was used in concert with another direct-acting mutagen, N-acetoxy-acetylaminofluorene (N-AcO-AAF). The isomeric product derived from sulfite and (+/-)-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (B[a]P-7,8-diol) is (+/-)-7,8,10-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene-9-sulfonate (B[a]PT-9-sulfonate). Like B[a]PT-10-sulfonate, B[a]PT-9-sulfonate is not mutagenic to strains TA97, TA98 and TA100. This sulfonate exhibited little enhancing activity with anti-BPDE over a 1-20 microM concentration range, but did enhance the mutagenic response of strain TA98 to 0.2 microM N-Aco-AAF by up to 128%. Sulfite, anti-BPDE and B[a]PT-sulfonates were also examined for the ability to induce a forward mutation at the hgprt locus (8-azaguanine resistance) in strains of S.typhimurium. Sulfite caused a marked enhancement of forward mutation due to anti-BPDE in both TA98 and TA100. Surprisingly, concurrent administration of B[a]PT-10-sulfonate with anti-BPDE did not increase the number of mutant colonies. The extensive conversion of anti-BPDE to B[a]PT-10-sulfonate under conditions where sulfite enhances diolepoxide mutagenicity, when coupled with this enhancement of diolepoxide mutagenicity by B[a]PT-10-sulfonate in the reverse mutation assay, supports this novel B[a]P derivative as a mediator of the sulfite-dependent enhancement of B[a]P genotoxicity. Determining why this enhancing effect was not seen when selecting for mutation at the hgprt locus of S.typhimurium will require further study.