Metabolites of tetracycline antibiotics have not previously been isolated from human excreta. It is now demonstrated that minocycline is the first tetracycline to be metabolized in man. A two-step liquid chromatographic procedure was used to isolate three metabolites from human urine. The principal metabolite was tentatively identified as 9-hydroxyminocycline by mass spectral and spectrophotometric analysis and comparison with a synthetic compound. The latter was prepared by incubation of minocycline in a modified Udenfriend system, simulating an enzymatic oxygenase. Two other major metabolites are probably mono-N-demethylated derivatives. Substantial amounts of 4-epiminocycline, which probably resulted from minocycline epimerization rather than biotransformation, were also present.