Whole tendons of chick embryos synthesize procollagens V which consist of three pro-alpha chains: pro-alpha 1'(V), pro-alpha 1(V) and pro-alpha 2(V). This report shows that while the pro-alpha 1'(V) chain is similar to the pro-alpha 1(V) chain in many respects, such as similar but not identical peptide maps, it also distinctly differs from it in size and in other ways. The new chain is denoted as pro-alpha 1' to indicate the relationship. We have failed to see conversion of one chain into the other and they are regarded as variants, although we do not know whether they are different transcripts of one gene or products of two closely related genes. The pro-alpha(V) chains are assembled into the disulfide-linked homotrimer (pro-alpha 1'(V))3 and the heterotrimer [(pro-alpha 1'(V)S-S-pro-alpha 2(V))pro-alpha 1(V)] and a smaller amount of [(pro-alpha 1(V)2pro-alpha 2(V)]. The pro-alpha 1'(V) chains are processed similarly to the pro-alpha 1(V) by the initial removal of the presumed carboxyl propeptide yielding p-alpha 1'(V) and then by reduction in the size of the noncollagenous, presumed amino propeptide to yield alpha 1'(V). A size difference between the alpha 1'(V) and alpha 1(V) series of molecules is demonstrated by velocity sedimentation and by electrophoretic mobility of the reduced molecules. This difference is ascribed to a difference in the size of the propeptides because after pepsin digestion the products of both series of molecules are the same size and electrophorese like alpha 1(V)(pepsin). The carboxyl propeptides of pro-alpha 1(V) and pro-alpha 1'(V) are the same size, but the amino propeptide of pro-alpha 1'(V) is smaller than that of pro-alpha 1(V). The amino propeptide of pro-alpha 1'(V) and p-alpha 1'(V) also lacks asparagine-linked complex carbohydrate, which is linked to propeptides of the p-alpha 1(V) and p-alpha 2(V) chains. Differences between the alpha 1(V) and alpha 1'(V) series of molecules remain in material synthesized in the presence of tunicamycin. Primary cultures of tendon cells synthesize procollagen V consisting of the above three chains, but the procollagen V made by cultured tendon sheath synovial cells predominantly contains [(pro-alpha 1(V))2pro-alpha 2(V)].