Of the family of P-450 cytochromes occurring in rabbit liver microsomes, only isozyme 3a (P-450ALC) is induced by alcohol administration and is effective in catalyzing the reaction: ethanol +02+NADPH+H+----acetaldehyde +2H2O+NADP+. As judged by immunochemical quantitation, P-450ALC is also induced in the animals by other diverse agents, including imidazole, trichloroethylene, acetone, pyrazole, and isoniazid. Evidence has been obtained for the occurrence of a protein immunochemically related to P-450ALC in human liver microsomes and of a similar alcohol-inducible protein in the rat and in the normal and alcohol dehydrogenase-deficient deer-mouse. P-450ALC catalyzes the activation of foreign compounds such as acetaminophen, various nitrosamines, and carbon tetrachloride and is therefore believed to play an important role in the enhanced toxicity of these substances accompanying alcohol administration.