1. Normal synovial membrane contains approximately equal proportions of two genetically distinct forms of collagen, types I and III. The proportion of these two collagens is unchanged in rheumatoid synovium but in addition a small amount of basement membrane collagen is present. Tissue culture of rheumatoid synovium confirms the synthesis of both type I and III collagens.
2. In young normal synovium both type I and type III collagens are stabilized by a reducible keto cross-link, which is replaced in adult tissue by an as yet unknown non-reducible cross-link. During the proliferation of the collagen in adult rheumatoid synovium a high proportion of the keto cross-link is present. This cross-link is not susceptible to cleavage by d-penicillamine, nor does the drug have any effect on the rate of synthesis in vitro. The mode of action of d-penicillamine in rheumatoid arthritis does not appear to involve a direct effect on the synovial membrane collagen.