Recent experiments with cell cultures of endothelial and smooth muscle cells derived from guinea pig thoracic aorta have shown the synthesis of several different types of collagen. Smooth muscle cells synthesize Type I and Type III collagens together with AB collagen and two recently described collagenous peptides of apparent M.W. 45,000 designated CP45a and CP45b. Endothelial cells synthesize only Type I collagen together with CP45a and CP45b. Future experiments will involve the isolation of procollagens for AB collagen and for CP45a and CP45b both from endothelial and smooth muscle cell cultures. Experiments will also be performed to isolate sufficient quantities of AB collagen and CP45a and CP45b from bovine aorta or chicken gizzard to perform additional biochemical characterizations.