Phenolic melanin precursors can be utilized for the development of anti-melanoma agents. The sulphur homologue of tyrosine, 4-S-cysteinylphenol (CP) and its decarboxylation product, 4-S-cysteaminylphenol (CAP) were shown to be substrates of melanoma tyrosinase, forming melanin-like pigment. Both, but in particular the M -C A P , exhibited a significant in vivo depigmenting effect. Here, we report on the in vivo anti-melanoma effect of 4-SXP, and 4-S-CAP and its N-acetyl derivative. In a previous in vitro study, it was shown that 4-S-CP and 4-S-CAP required a catalytic amount of dopa for optimal mammalian tyrosinase activity. To enhance the potential anti-melanoma effect off these two compounds, Ldopa and a decarboxylase inhibitor (carbidopa) were given concomitantly. We found that 4-S-CAP showed a significant growth inhibition of B16 melanoma inoculated S.C. into CS7BU6J mice. The anti-melanoma effect was increased significantly by combination of L-dopa and carbidopa. In addition, we tested the in vivo anti-melanoma effect of an Nacetyl derivative of 4-S-CAP (N-Ac-4-S-CAP). We found that N-Ac-4-S-CAP was the tyrosinase substrate and potent inhibitor of melanoma growth. N-acetyl 4-S-CAP showed a marked increase in water solubility. We suggest that N-Ac-4-S-CAP may prove to be a valuable model for the development of anti-melanoma agent using a metabolic pathway of melanin synthesis.