This study deals with the role of glutathione transferase (GST)-mediated conjugation of (+)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-oxy-7,8,9,10- tetrahydrobenzoapyrene (BPDE) in two mammalian cell lines, human mammary carcinoma cells (MCF-7) and rat hepatoma cells (H4IIE), in relation to their capacity to metabolize (-)-trans-7,8-dihydroxy-7,8-dihydrobenzoapyrene (-)-BP-7,8-diol to products that induce mutations in co-cultivated V79 cells. Both MCF-7 and H4IIE cells metabolized (-)-BP-7,8-diol to BPDE, but mutations in co-cultivated V79 cells were only detected with MCF-7 cells. However, depletion of glutathione (GSH) in H4IIE cells increased the mutagenicity of (-)-BP-7,8-diol to a similar level to that found with MCF-7 cells. Measurements of GST activity using GSH and post-microsomal supernatants from H4IIE, V79 and MCF-7 cells indicated a substantial difference in conjugation capacity. Although preparations from all three cell-lines showed GST activity with 1-chloro-2,4-dinitrobenzene as the substrate, GST activity towards BPDE could only be detected in supernatants from H4IIE cells. This is consistent with the presence of GST 7-7 an isoenzyme highly efficient in catalysing BPDE-GSH conjugation. The difference in GSH-conjugation activity towards BPDE was confirmed using intact H4IIE and MCF-7 cells in culture. These results indicate that GSH-conjugation plays a pivotal role in mutagenesis induced by polycyclic aromatic hydrocarbons (PAH). Accordingly, a deficiency in GSH-conjugation capacity may be regarded as one important factor in defining a target cell population with an increased risk for tumour initiation following exposure to PAH.