Samples of (±)-3,4-dihydroxyphenylalanine deuteriated at the α-, β-, 2-, 5-, and 6-positions were each converted into melanin, both by autoxidation and enzymically, and the incorporation of deuterium into these melanins was measured. The results were interpreted in terms of an outline structure suggested for dopa-melanin on the basis of other experiments (Part X); and the relative numbers of polymeric linkages at different positions of the polymeric units were estimated. No evidence was found that enzymic dopa-melanin was fundamentally different from the autoxidative melanin. Dopa-melanin, prepared in vitro, appears to be an irregular polymer, containing a number of different types of unit, linked in various ways.