ESR spectra of synthetic melanins formed from dopa and from 5-S-cysteinyldopa and of melanins isolated from human melanomas were studied. Alkali-soluble melanin synthesized from cysteinyldopa showed a signal similar to that of insoluble melanin synthesized from dopa, but also a simple fine structure. Trichochromes C and F synthesized from 5-S-cysteinyldopa had a broader signal than dopamelanin, and in addition a hyperfine structure that could be explained by the presence of manganous ions. An insoluble eumelanin from a human melanoma with a sulphur content of 5% showed an ESR spectrum consisting of one line like dopamelanin. An alkali-soluble phaeomelanin from a human melanoma with a sulphur content of 10% showed a very broad signal with a hyperfine structure that could be explained by the presence of manganous ions.