The present study examined the utility of an immunoblot method for quantitation of cytochrome P450 isozymes in archived liver samples as a bioassay of exposure to halogenated hydrocarbons. Hepatic microsomes were prepared from 44 archived polar bear (Ursus maritimus) liver homogenates that had been stored at approximately -40 degrees C for 9-10 years and analyzed on blots probed with antibodies to rat cytochromes P450 1A1 and P450 2B1. The results revealed a positive correlation between cytochrome P450 1A and total polychlorinated biphenyl (PCB) levels in the archived liver samples, suggesting that cytochrome P450 1A was induced in polar bears by environmental exposure to PCBs.