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Summary To detect a foreign collagen in a tissue on a substantially idemtied protein, a radio-tracer technique appeared ideal. The method is only valuable if the introduction of the tram element into the micro molecule does not modify its properties. The first collagen used was tagged by the introduction of C1* into the peptide chain from marked proline by biosynthesis. A 0.3% concentration in sorbic acid solution was applied to the back of the animal and the penetration followed by measuring the activity of tissue sections. No radioactivity was found in the interior of the deinnir after periods of 15, 30,90 and 120 min after application. The method of biosynthetic marking was abandoned as the activity of the protein was considered to be insufficient. The technique of the incorporation of 1(125) in the tyrosine was used to obtain a greater specific activity without modifying the active principle. This chemical treatment resulted from the oxidation of tagged iodide in the presence of collagen. The protein was purified by chromatography and successive dialyses and diluted in unlabelled collagen for subsequent manipulation. Skin applications were made in sorbic acid solution at 0.3% and at 10% in a cosmetic cream. Skin penetration forthe same periods as previously were followed by the same sectionning technique. The penetration reached a maximum at the end of 15 min and was greater for the cream than for the solution. Metabolic studies showed that 15 days after application the residual radioactivity was present mainly in the lymphatic glands and in the skin. Treated dcin showed more activity than untreated areas. At the end of 10 days the excretion of radioactivity had dropped to very low levels. Chromatography on molecular sieve of the collagen extracted from the deeper layers on the skin indicated that the largest part of the radio-activity originated in peptide chains.