Activities of four intracellular enzymes of collagen biosynthesis--prolyl hydroxylase, lysyl hydroxylase, collagen galactosyltransferase, and collagen glucosyltransferase--were demonstrated in human platelets, and the presence of prolyl hydroxylase protein was further demonstrated by direct radioimmunoassay. The ratio of the specific activities of the four enzymes in the four enzymes in the human platelet extract to those in human adult skin extract varied from about 0.1 to 1, the lowest relative activity being found with prolyl hydroxylase and the highest with collagen glucosyltransferase. Only a very small amount of prolyl hydroxylase protein, probably 1%, was in the form of the active enzyme tetramer. The collagen glucosyltransferase from human platelets readily glucosylated galactosylhydroxylysine in denatured collagen, but did not glucosylate native collagen. Also, native collagen did not act as an inhibitor of the glucosylation reaction. Therefore, platelet collagen glucosyltransferase cannot form either an enzyme--substrate complex or an enzyme--inhibitor complex with native collagen. The results thus argue against the theory which maintains that platelet collagen glucosyltransferase is involved in collagen--platelet adhesion.