Small amounts of a number of metal ions were found in melanins obtained from natural sources [B-16 melanoma tissue of C56 black mouse], and 140Ba and 226Ra were found to deposit in tissues containing melanin. Some aspects of the chemical nature of the incorporation were investigated with radio-nuclides of high specific activity. The uptake of metal ions by synthetic melanin from a physiological buffer increased with decreasing hydrated ionic radius, decreased with decreasing pH, and decreased in the presence of chelating agents. Replacement of melanin-bound Ra++ with H+ demonstrated the reversiblity of the reaction. Comparative studies with synthetic melanin and natural melanin plus its protein showed that the role of the protein in the reactions is small. Both the reaction chracteristics and the chemical structures of melanin and of weak acid cation exchangers are similar, which suggests that metal ions and melanin interact by ion exchange.