Exposure to crude oil and certain petroleum products can be a serious health hazard. Clarified slurry oil (CSO) is a complex mixture of hydrocarbons derived from the processing of crude oil, and is a known systemic and developmental toxicant, mutagen, and carcinogen. In the present study, CSO and two crude oils, Belridge heavy crude oil (BHCO) and Lost Hills light crude oil (LHLCO), were examined for their estrogenic and antiestrogenic properties in a human breast-cancer cell (MCF-7) assay. The MCF-7 focus assay is based on postconfluent cell growth and tissue restructuring, measured as the postconfluent development of multicellular nodules or foci. The mutagenicity indices of BHCO and LHLCO also were determined in a modified Ames Salmonella assay. Oil samples were prepared in dimethyl sulfoxide, resulting in extraction of virtually all of the aromatic compounds including the sulfur- and nitrogen-substituted three- to seven-ring polycyclic aromatic compounds comprising 62.2% of the CSO, 9% of the BHCO, and 2% of the LHLCO by total weight. None of the three samples was estrogenic in the MCF-7 focus assay. In contrast, all of the samples were antiestrogenic; that is, they inhibited the development of foci induced by 1 nM 17beta-estradiol (E2). The potencies of the oil samples for both antiestrogenicity and mutagenicity were correlated with the percent of polycyclic aromatic compounds they contained. Two potential mechanisms for the observed antiestrogenicity were examined. Radiometric analysis of the catabolism of [3H]E2 in MCF-7 cell cultures demonstrated that all three samples increased catabolism of E2. Results from a whole-cell estrogen-receptor (ER) binding assay suggested that metabolites of compounds in the oil samples might have competed with [3H]E2 for ER in the MCF-7 cultures. Thus the antiestrogenicity of the oil samples may occur through at least two mechanisms, increased catabolism of E2 and antagonistic binding to ER.