SYNTHESIS of collagen-a principal product of fibroblasts-occurs in various cultured cells of fibroblastic and non-fibroblastic origin1-5. When tested in conditions allowing the maximal rate of collagen formation, virus-transformed cells of fibroblastic origin produce less collagen per unit of protein synthesized than the non-transformed cells from which they were derived4. Some cells of non-fibroblastic origin synthesize collagen at about 5 per cent of the rate characteristic of fibroblasts, while in others collagen synthesis is virtually absent6. Thus synthesis of collagen is decreased (a) in virus-transformed cells of fibroblastic origin and (b) in cells of non-fibroblastic origin. Collagen is a glycoprotein and contains a glucosyl 1-2-galactose disaccharide joined in O-glycosidic linkage to hydroxylysine8. During an investigation of the biosynthesis of this unit, we isolated and purified an enzyme from embryonic guinea-pig which transfers 14C-glucose from 14C-UDP-glucose to a receptor prepared from guinea-pig skin collagen by mild acid hydrolysis7. The assay is highly specific for collagen presumably because of the requirement for the proximity of hydroxylysine. Using this assay we have shown that the collagen-glucosyl transferase is present in cultured cells and the activity in each cell parallels the rate of collagen synthesis occurring within the cell line.