A technique is proposed for the biological monitoring of pollutants in aquatic environments by use of a complementary DNA (cDNA) probe. The induction of hepatic cytochrome P(1)450 mRNA has been investigated utilizing pfP(1)450-3', a 3'-specific 1.5 kb cDNA clone derived from 3-methylcholanthrene-inducible mRNA of rainbow trout. A time course of induction of both the hybridizable mRNA and hepatic monooxygenase catalytic activity in rainbow trout with a known inducer in fish, beta-naphthoflavone, was studied. The cDNA probe was also shown to hybridize with induced mRNA of brook trout, scup, garter snake, painted turtle, and rat demonstrating the suitability of the probe for examining induction of mRNA in various species. The results of these experiments suggest that the cDNA probe may be useful as a biological monitoring tool for determining the presence and effects of chemical pollutants which are inducers of hepatic microsomal monooxygenase activity. The probe may have the potential to be applied as an early warning system in the monitoring of water quality.