Urinary 5-S-cysteinyldopa (5-S-CD) has been used as a biochemical marker of melanoma metastasis. A method was developed for determining the eumelanin-related metabolites 5(6)-hydroxy-6(5)-methyoxyindole-2-carboxylic acids (5H6MI2C and 6H5MI2C) in small volumes of serum. We compared these indoles and 5-S-CD regarding the correlation of their production in melanoma, circulation in blood, and excretion in urine, with the weight of highly pigmented, B16 mouse melanoma. An excellent correlation was found between the serum concentration of 5H6MI2C + 6H5MI2C (r = 0.92) and 5-S-CD (r = 0.89) and tumor weight. However, the urinary excretion of 5H6MI2C + 6H5MI2C and 5-S-CD did not show any significant correlation. These results suggest that 5H6MI2C + 6H5MI2C and 5-S-CD in serum may better reflect melanoma progression than those in urine. Furthermore, comparison of the contents of these melanin-related metabolites between highly pigmented and less pigmented B16 melanomas suggests that 5-S-CD may be accumulated in pigmented melanoma by virtue of binding to melanin and that catechol-O-methyltransferase (COMT) may play a regulatory role in pigmentation.