Metabolism of delta 1-tetrahydrocannabinol (delta 1-THC) in humans, has been studied in vitro with the 10,000g supernatant fraction of three human livers. One of the livers (H-13) showed very high enzyme activity compared to the other two. The metabolites were characterized by their mass spectra and in most cases by comparison of their mass spectra and chromatographic properties with those of available synthetic references. The metabolic pattern was similar in all three livers, with 7-hydroxy-delta 1-THC as the most abundant metabolite. Furthermore, they all formed compounds oxygenated in the 6-position. Sidechain oxygenation was most pronounced with H-13, whereas trace mounts of 7-hydroxy-epoxide were formed by livers H-13 and H-15.