A sulfur-containing amino acid compound, S-allyl cysteine (SAC), derived from garlic extract inhibited proliferation of nine human and a murine melanoma cell line in a dose-dependent manner (1.2–10 mM) assessed by a [3H]thy-midine incorporation assay. Three control human lymphoblastoid cell lines were not inhibited by SAC concentrations < 5 mM. Four human melanoma cell lines in a soft-agar assay also showed dose-dependent inhibition of colony formation by SAC. Melanin content was increased up to 95% compared to the same untreated cell lines in these four human melanoma and two B16 murine melanoma sublines. Expression of cell surface gangliosides, cellular-differentiation and transformation markers, decreased after SAC treatment. Significant morphological changes including ‘flattening and/or dendritic-like elongations’ were also observed. Thus SAC inhibited cellular growth and proliferation and modulated major cell differentiation markers of melanoma.