Melanoma tissues produce not only 5-S-cysteinyldopa (5-S-CD), a pheomelanin precursor, but also 5,6-dihydroxyindoles, eumelanin precursors. We compared 5-S-CD and eumelanin-related metabolites regarding the correlation of their production in melanoma and excretion in urine, versus the weight of B16 mouse melanoma. It was found that B16 melanoma tissues produce 5,6-dihydroxyindole-2-carboxylic acid and its two O-methyl derivatives, the latter of which are partly conjugated with sulphuric acid and then excreted. These indolic metabolites are referred to as total indoles. Both total indoles and 5-S-CD in melanoma and in urine correlated well with each other and reflected the tumour weight. Total indoles had a four-fold greater level of excretion than 5-S-CD, suggesting that the urinary excretion of total indoles is a quantitatively more significant marker of melanoma progression in this melanoma.