Specimens of the feral mouse species Reithrodontomys fulvescens trapped from a polychlorinated biphenyl (PCB)-contaminated field location had hepatic ethoxyresorufin (ETR) O-dealkylase activities and immunoreactive CYP1A protein contents which were two- to threefold higher than those measured in animals of the same species and sex collected from non PCB-contaminated reference sites. Specimens with hepatic ETR O-dealkylase activities differing by as little as 50% could readily be assigned as originating from the PCB or reference sites by the use of a specific chemical inhibitor of cytochrome P450IA (CYP1A). The relative levels of ETR O-dealkylase activity in R. fulvescens significantly correlated with hepatic PCB burdens (r = 0.819, P less than 0.01). When the magnitudes of the induced ETR O-dealkylase activities corresponding to given hepatic PCB burdens were compared between the feral animals, F344/NCr rats (Rattus norvegicus) or B6C3F1 mice (Mus musculus) exposed in the laboratory to dietary Aroclor 1254, the order of sensitivity to the inducing effects of PCBs were F344/NCr rat greater than B6C3F1 mouse greater than R. fulvescens.