The formation of collagen in bone, tendon, cartilage and some other tissues is being studied. Freshly excised tissue from chick embryos, or from other sources, and cells cultured from such tissues, are labeled in vitro by incubation with radioactive proline and with other small molecular weight precursors of collagen. The radioactive collagen synthesized during this time is extracted and analysed by a variety of biochemical procedures. Collagen is made initially as a large molecule, called procollagen, and by means of pulse-chase time studies we are following how this procollagen is exported from cells and sequentially cleaved to the final product, collagen, and how these collagen molecules associate to form fibers. BIBLIOGRAPHIC REFERENCES: Fessler, L.I., Morris, N.P. and Fessler, J.H., "Procollagen: Biological Scission of Amino and Carboxyl Extension Peptides," Proc. Natl. Acad. Sci. USA 72, 4905-4909, (1975). Fessler, J.H., and Fessler, L.I., "Procollagens of Different Types and Their Processing in Chick Tendon, Cartilage and Aorta," Federation Proceedings 35 (1976).