We conducted a review of literature from the MEDLINE data base (1966-January 1997), and bibliographies of published articles, reviews, and letters to classify enzyme induction of warfarin metabolism interactions by onset, extent, and offset. Ten hepatic microsomal enzyme agents were assessed. Likelihood of interaction was based on the strength of supporting literature. Enzyme induction of warfarin metabolism by rifampin and barbiturates is considered likely, although the characteristics of the interactions are different. An interaction is probable with carbamazepine, griseofulvin, aminoglutethimide, nafcillin, and dicloxacillin. A suspected interaction may occur with smoking and long-term alcohol consumption. Ingestion of a small amount of alcohol is unlikely to interact with warfarin. The effect of phenytoin on warfarin metabolism is unpredictable. Anticipation of the expected time course and extent of interaction may allow for better therapeutic decisions and decrease the chance of inappropriate anticoagulation with its potential for complications.