This work studies the phenotype changes, relating to pigment expression, of a human melanoma cell line. The phenotypic instabilities and proliferation rates are correlated with the production and release in the cell culture medium of active oxygen species and melanin synthesis intermediates. The proliferation rates versus L-tyrosine concentration in the culture media are investigated: a decrease is found when high L-tyrosine is added to the medium. This would be consistent with the release of cytotoxic and/or genotoxic species by melanoma cells. The morphology of melanoma melanosomes is coherent with the leakage of cytotoxic and genotoxic species produced during melanin synthesis.