SINCE collagen is rich in hydroxyproline and practically devoid of tyrosine, alterations in the amounts of these amino acids should afford a good indication of changes in skin collagen content which might be induced by the application of carcinogens. Hamer and Marchant (1957) in experiments carried out in this laboratory found little change in the tyrosine content of the skins of mice, a slight decrease in the hydroxyproline content of skin from male mice but little change in female skin, after 12 weekly applications of 0 3 per cent methylcholanthrene in acetone. From these observations, in conjunction with parallel extractions of the collagen and acid-soluble (procollagen) fractions, and analyses of the polysaccharide and ground-substance components, they concluded that physical changes in the state of association of the collagen rather than changes in chemical composition, are responsible for the effects of carcinogens that have been observed histologically (Orr, 1938; Vernoni, 1951). UTnfortunately values at intermediate periods of treatment were not determined
