Hydroxylysine (Hyl) glycosides were determined both on the collagen produced by in-vitro cultures of human skin fibroblasts and pig articular chondrocytes and on the insoluble collagen extracted from bovine cornea and sclera. The disorganized collagen present in the culture medium showed a Hyl di- to mono-glycoside ratio markedly higher than the molecules extracted from the cell layer, where electron microscopy investigations demonstrated the real presence of collagen fibres. Moreover in bovine cornea the insoluble collagen showed a ratio Hyl di- to mono-glycoside significantly higher than in sclera, which, on the contrary, possesses fibres with larger mean diameter. The possible conversion of Hyl diglycoside to monoglycoside was suggested by the demonstration of an alpha (1----2) glucosidase activity on Hyl diglycoside , in an enzyme extract from cultured human skin fibroblasts. A role for the processes of collagen glycosylation and deglycosylation was thus considered.