V79 Chinese hamster cells have previously been shown to lack the capacity to detoxify the mutagenic and carcinogenic compound (+)-anti-benzo[a]pyrene 7,8-dihydrodiol 9,10-epoxide [(+)-anti-BPDE] by Pi class glutathione transferase (GSTPi)-catalysed conjugation with GSH, although these cells contain such an enzyme [Romert, Dock, Jenssen and Jernström (1989) Carcinogenesis 10, 1701-1707; Swedmark, Romert, Morgenstern and Jenssen (1992) Carcinogenesis 13, 1719-1723; Swedmark and Jenssen (1994) Gene 139, 251-256]. Previous findings also indicate that these results do not depend on an inactive GSTPi enzyme, since V79 cells conjugate 1-chloro-2, 4-dinitrobenzene (CDNB) with GSH, but more likely on (a) factor(s) that inhibit(s) V79 GSTPi selectively [Swedmark, Jernström and Jenssen (1996) Biochem. J. 318, 533-538]. The present study demonstrates that both human and V79 recombinant GSTPi enzymes are inhibited with respect to conjugating (+)-anti-BPDE, but not CDNB, after pre-incubation with V79-cell extract, but not with MCF-7-cell extract. In addition, it was found that the inhibition is dependent on the amount of cell extract present and that the factor(s) is heat-resistant and has a molecular mass of less than 10 kDa, suggesting that the factor(s) is (are) non-proteinaceous in nature.