Stromal cells from mouse endometrium, E041 cells, at passages 21, 23 and 26 were metabolically active towards 7,12-dimethylbenz[a]anthracene (DMBA). The total DMBA-metabolizing activity of E041 cells was preferentially increased by benz[a]anthracene (BA) relative to 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD) treatment (7-fold by BA and 4-fold by TCDD), but the relative proportions of DMBA metabolites formed remained unchanged. Profiles of DMBA metabolites generated from E041 cell microsomes were very different from that of mouse P4501A1 but exhibited similarities to that of P450EF, the polycyclic aromatic hydrocarbon (PAH)-inducible cytochrome P450 in the mouse embryo fibroblast cell line C3H10T1/2 (10T1/2). Notably, both induced and uninduced E041 cell microsomes were very effective in the formation of the proximate carcinogen DMBA-3,4-dihydrodiol (15-20% of total) and DMBA-10,11-dihydrodiol (13-18% of total) but ineffective in forming the 7-hydroxymethyl derivative of DMBA (< 1% of total). Antibodies to P450EF completely inhibited the DMBA-metabolizing activities of both induced and uninduced E041 cell microsomes, with an effectiveness similar to that in microsomes prepared from identically treated 10T1/2 cells. Anti-P4501A1 had no inhibitory effect on DMBA metabolism by either induced or uninduced E041 cell microsomes. Total DMBA-metabolizing activities in BA- and TCDD-induced E041 cells were consistently 2-fold lower compared to those in similarly treated 10T1/2 cells. In addition, both induced and uninduced E041 cell microsomes formed a lower proportion of DMBA dihydrodiols relative to phenols in comparison to identically treated 10T1/2 cell microsomes (0.5 versus 1). Addition of exogenous epoxide hydrolase to E041 cell microsomes resulted in a product distribution indistinguishable from that in 10T1/2 cells. Immunoblots demonstrated 5-fold lower levels of epoxide hydrolase in E041 cell microsomes compared to 10T1/2 cell microsomes. Anti-P450EF immunoblotted a 55 kd protein in E041 cell microsomes that was induced 14-fold by BA and 6-fold by TCDD, thus paralleling the increases in the respective DMBA metabolism. It is therefore concluded that following PAH exposure endometrial stromal cells express the novel PAH-inducible mouse embryo fibroblast P450 and fail to express P4501A1.