Cysteine is known to be involved in pheomelanin synthesis. Available evidence, however, indicates that glutathione may also play an important role in melanogenesis. This in vitro study clarifies how cysteine and glutathione participate in melanogenesis. Melanins were prepared by tyrosinase oxidation of 3,4-dihydroxyphenylalanine (dopa) with various amounts of cysteine or glutathione, and were subjected to HCl hydrolysis. Synthetic melanins and the hydrolyzed melanins gave N/S molar ratios corresponding to those calculated from the ratio of dopa to cysteine or glutathione. On hydriodic acid hydrolysis, dopa plus cysteine-melanins and dopa plus glutathione-melanins gave aminohydroxyphenylalanine and cysteine, respectively, the yields of which were proportional to the sulfur content. The results indicate that cysteine is integrated into benzothiazine units (pheomelanins) while glutathione is connected to dihydroxyindole units (eumelanins) with the retention of glutathione moiety. Analysis of natural melanins, prepared by HCl hydrolysis of Sepia, B16, and Harding-Passey melanosomes, indicates that sulfur (0.4-1.4%) in these natural melanins may be derived artificially from the reaction of melanins with cysteine or cystine in the course of HCl hydrolysis.