Most metabolites of caffeic acid that are found in the urine of man and experimental animals arise as the result of reactions of the intestinal microflora of the host. This conclusion is now confirmed by the observation that O-methylation, which can be attributed to mammalian enzymes, is the only reaction sustained by caffeic acid in germfree rats. When the germfree rats are selectively infected by one or more bacteria characteristic of the gastrointestinal tract of rodents, the feeding of caffeic acid leads to the appearance in the urine of additional metabolites of caffeic acid. The apparent ability of these bacteria in the host to transform caffeic acid does not necessarily correlate with the transformations of caffeic acid that are demonstrable when the same bacteria are cultivated on artificial media.