It has been shown that melanin has the properties of a polyanion and may in vivo and in vitro bind inorganic cations and drugs which are positively charged at physiological pH by a cation-exchange mechanism. In the present study, we explored if the organic aliphatic polycation spermidine would bind to melanin in vivo after administration of 14C-spermidine to pigmented mice and in vitro at incubation with pigment from beef-eyes. The results showed a high labelling of the pigmented tissues in the mice after the administration of 14C-spermidine. At long survival intervals, the radioactivity in the melanin was higher than in any other tissue. A strong melanin affinity of 14C-spermidine was found in vitro. An analysis of the binding by the method of Scatchard showed that the data could be best fitted by the assumption of two classes of binding sites. The in vivo bound material could be displaced by in vitro incubation in solutions containing inorganic cation-chloride salts or HCl and HCl was also very effective in inhibiting the melanin-binding of 14C-spermidine in vitro. The results indicate that an electrostatic interaction between spermidine and melanin will occur both in vivo and in vitro.