Cultures of adult mouse epidermal keratinocytes (MEKs) were utilized to determine whether the metabolism and metabolic activation of polycyclic aromatic hydrocarbons varied as a function of extracellular calcium (Ca2+) concentration. MEKs grown in low Ca2+-containing medium (0.05-0.10 mM) maintain basal cell morphology and proliferate while increasing the Ca2+ concentration in the medium to 1.2-1.4 mM signals the cells to undergo terminal differentiation. Relative to cultures of undifferentiated MEKs (low Ca2+), cultures of differentiated MEKs that had been switched to high Ca2+ medium 48 h prior to treatment with benzo(a)-pyrene [B(a)P] and 7,12-dimethylbenz(a)anthracene (DMBA) exhibited more rapid overall metabolism of both hydrocarbons. The greatest differences in the metabolism of B(a)P and DMBA between the two types of cultures occurred after a 3-6-h lag period. In addition, the levels of DNA-adducts formed from B(a)P and DMBA after a 24-h exposure to the hydrocarbon were 4- and 3-fold higher respectively, in cultures of differentiated MEKs (high Ca2+). Higher levels of mutagenesis and cytotoxicity were also observed in cocultures of Chinese hamster lung V-79 cells and MEKs that had been switched to high Ca2+-containing medium. In cocultures treated with the hydrocarbons at the time of Ca2+ shift, several hours elapsed before differences in mutagenesis were apparent between high and low Ca2+-containing cultures. This lag period was eliminated if the MEKs were switched to high Ca2+ medium 24 h prior to exposure to DMBA. Based on the present data, we propose that the expression and inducibility of certain enzyme activities involved in the metabolism of B(a)P and DMBA by cultured MEKs is regulated by the extracellular Ca2+ concentration and possibly the Ca2+-induced differentiation of MEKs.