OBJECTIVE Glutathione (GSH) was found to occur in relatively high concentrations in melanoma cells. The purpose of this study was to test the possible cytotoxic effects of an artificial decrease of the elevated GSH level. METHODS The tests were made in vitro and in vivo. In the former case, a total of 11 rodent and human cell lines were studied of which seven were derived from melanomas. After treatment with buthionine sulfoximine (BSO), the decrease of GSH content of the cells and their clonogenic survival was determined. In the in vivo system, single cell suspensions of a subline of the B16 mouse melanoma were injected intravenously into immunocompetent and preirradiated recipients which were subsequently treated with BSO intraperitoneally. Survival time, formation of lung colonies and the weight of metastatic tumor mass in the lungs were the criteria of the BSO effect on the tumor cells. RESULTS The decrease of the GSH level by BSO was associated with impaired clonogenic survival of the melanoma cells in vitro. Nonmelanoma cells were less affected. BSO treatment of mice inoculated intravenously with melanoma cells resulted in prolonged survival of the animals and impaired metastatic spread of the tumor cells. CONCLUSIONS Melanoma cells are particularly sensitive to disturbance of GSH metabolism by treatment with BSO. In view of this selective cytotoxicity of BSO, treatment with this substance may afford a promising therapeutic potential for melanoma.