Phenol-water extracted rheumatoid synovial fluids and synovial fluid leukocytes contain an antigen immunologically identical to the Proprionibacterium group bacteria. The antigen was identified by counter-immunoelectrophoresis in 70% of rheumatoid synovial fluid leukocyte pellets and in 60% of rheumatoid synovial fluids. It was also present in 6% of nonrheumatoid fluids and in 22% of nonrheumatoid inflammatory fluid leukocytes. Antigen was not detectable in synovial samples before extraction. Synovial and bacterial antigens were further purified by proteolytic digestion and Sepharose 4B column chromatography. Biochemical and enzymatic studies of bacterial and synovial antigens were similar and consistent with a high molecular weight polysaccharide. Serum antibody to bacterial and synovial antigens was significantly less frequent in rheumatoid sera than in normal controls. The significance of demonstrating a bacterial polysaccharide primarily in rheumatoid synovial effusions is discussed.