The major cytochrome P450 (P450EF) in the mouse embryo fibroblast C3H/10T1/2CL8 (10T1/2) cell line, which is very active in polycyclic aromatic hydrocarbon metabolism, is immunologically distinct from known P450 families but shares homology with an adrenocorticotropin hormone-regulated P450 from rat adrenal glands (P450RAP). P450EF is more effectively induced by benz[a]anthracene (BA) than by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which is anomalous for aryl hydrocarbon receptor (AhR)-mediated transcriptional activation. Evidence is presented here that induction of P450EF is consistent with mediation by the AhR but also involves an additional selective stabilization of P450EF by BA. P450EF-specific mRNA was measured by in vitro translation of 10T1/2 mRNA and subsequent immunoprecipitation with antibodies that recognize P450EF. P450EF mRNA was equally stimulated (> 10-fold) by BA (10 microM) and TCDD (10 nM) after 6 hr of induction in 10T1/2 cells. This equal stimulation of P450EF by BA and TCDD is consistent with transcriptional activation of the gene by the AhR. BA induction of mRNA declined 3-fold between 6 and 18 hr, due to metabolism of BA. Steady state P450EF mRNA levels declined quickly once this stimulation was removed, whereas total P450EF protein levels, measured by immunoblotting, continued to increase. During a 6-hr inhibition of protein synthesis with cycloheximide, both total P450EF and functional cytochrome, measured by polycyclic aromatic hydrocarbon metabolism, decreased by 60% in uninduced and TCDD-induced transformed 10T1/2 cells. This is consistent with relatively rapid degradation of P450EF (t1/2 = 4 hr). No such decline was seen when BA was present, indicating a stabilization of P450EF, which can explain the additional effectiveness of BA in enhancing the level of P450EF.