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Melanosomes isolated from human, horse and Harding-Passey mouse melanomas, from human hair and from bovine eye pigment tissues were hydrolyzed with 6N HCl 24hours at 110 °C. An electron microscopic comparison of hydrolyzed and untreated melanosomes showed that the hydrolyzed melanosomes retained their size, shape, and, to a great extent, their ultrastructural features. The results 1/suggest that melanin constitutes the rigid framework of the particles providing them with unusual resistibility; 2/raise the question of the nature of their lysosomal degradation/reported by some authors/ if extreme acid hydrolysis has remained without any effect. Melanosomes are special subcellular particles of pigment cells ( Seiji et al., 1961 ) originating by apposition of polymerizing melanin precursors on primary protein polymer — either on premelanosome matrix protein exhibiting tyrosinase activity or on matrix protein under tyrosinase catalysis. The synthetized melanoprotein complex is a tough substance, which represents the major melanosome component. Unlike the biosynthesis, the melanosome degradation has remained obscure. Electron microscopy revealed that in vivo melanosomes are disintegrated in lysosomes ( Mishima, 1966; Kawamura et al., 1967; Sato et al.,1969; Olson et al., 1970; Wolff and Honigsmann, 1972; Wolff, 1974). The in vitro model experiments confirmed that the protein moiety of melanosomes could be hydrolyzed by lysosomal enzymes ( Ohtaki, 1970; Ohtaki and Seiji, 1971; Saito and Seiji, 1973 ), however, the melanin moiety appeared to be very resistant to lysosomal digestion. In this study the isolated melanosomes were, therefore, exposed to extreme hydrolytic treatment — acid hydrolysis and its influence was studied by means of electron microscopy.