Animal and bacteriological techniques have been developed for clarifying the role of the flora in the metabolism of drugs and other exogenous compounds. In general the flora tends to catalyze reductive and hydrolytic reactions, some of which reverse the detoxification reactions normally occurring in the liver. These reactions and others have been implicated in the pharamacological or toxicological action of exogenous compounds. Only in a few instances, however, have practical consequences of these reactions been documented. The major challenge at present is to develop methods capable of further defining the implications of reactions due to the flora.