Fibroblast‐like cells from synovial tissue obtained during arthroscopy in 4 young adults with recent knee trauma were biosynthetically labeled with 35S‐methionine, and protein production was quantitated by immunoprecipitation and sodium dodecyl sulfate—polyacrylamide gel electrophoresis. Synovial fibroblast‐like cells synthesized C1r, C1s, C1 inhibitor, C2, C3, factor B, and factor H, all with the same sizes and subunit structures as the proteins synthesized in skin fibroblasts. The capacity to synthesize these proteins was not lost with passages or freeze‐thawing. Gamma‐interferon stimulation increased synthesis of all 7 proteins. Lipopolysaccharide increased synthesis of only C3 and factor B. Unlike in whole rheumatoid tissue, C4 and C5 were not detected. Synovial lining cells may be an important source of local complement for participation in local defense or development of pathologic states.