In this study, we measured the content and activity of CYP1A in liver of winter flounder, Pleuronectes americanus, populations over a large geographic region ranging from New York , U.S.A., to New Brunswick, Canada, and investigated the relationship between content and activity of CYP1A and the concentration of PCBs in livers of flounder from sites along the New York and New England coast and Georges Bank. This comparison was the outgrowth of our search for flounder that would show at most slight indication of exposure to Ah receptor agonists or other contaminants. Locations of such 'reference' populations would be useful in helping to establish the etiology of hepatic neoplastic and other diseases of flounder living in the vicinity of contaminated sediments. Flounders in some locales bear neoplastic and other histological diseases that are thought to derive from environmental contamination.