An analytical procedure has been developed for the determination of the 18 PCB congeners which are inducers of methylcholanthrene-like mixed function oxidase activity in animals and include the most toxic PCBs. Determinations of the toxic PCB congeners in samples of eggs of predatory fish and piscivorous birds of the Great Lakes and in Aroclor mixtures demonstrate that the apparent toxic potency of PCB residues in these samples is dominated by two congeners, 3,3',4,4',5- and 2,3,3',4,4'-pentachlorobiphenyl. Furthermore, the analyses demonstrate that the potential toxicity of PCB residues can increase 5 to 10 fold as they reach upper levels of aquatic food chains and most often exceed the potential toxicity of chlorinated dibenzodioxins and furans in higher animals even in environments highly contaminated by the latter compounds.