Phenol-water extracted rheumatoid synovial fluids and synovial fluid leukocytes contain an antigen immunologically identical to the Proprionibucterium group bacteria. The antigen was identified by counterimmunoelectrophoresis in 70% of rheumatoid synovial fluid leukocyte pellets and in 60% of rheumatoid synovial fluids. It was also present in 6% of nonrhellmatoid fluids and in 22% of nonrheumatojd 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.From the Division of Rheumatology, Department of MediWe have previously reported the isolation of a phenol-water extractable antigen immunologically identical to a Proprionibacterium acnes antigen in rheumatoid (RA) synovial fluid and synovial leukocytes (1,2). Antigen was found only after extraction, suggesting that it is present in vivo in a form not available for immunologic identification. Complement-fixing antibody to P ucnes was demonstrated in a significantly lower incidence of RA than control sera (1). This finding raised a number of possibilities including rapid utilization of antibody or a low level of recognition of antigen.This study reports a more sensitive assay method for detection of P acnes antigen in synovial fluid and synovial fluid leukocytes by counterimmunoelectrophoresis (CIE). With this system the frequency of isolation of P acnes antigen in RA samples is considerably higher than in our previous studies. Since free P acnes antigen has not been detected in synovial fluids or sera by CIE, a more sensitive method was studied by '251 radioautography.Antibody to P acnes antigen was also determined by CIE in sera and synovial fluid from patients with RA and various control groups. These results are compared Submitted for publication November 27, 1978; accepted in to our previous studies using a complement-fixation method for determination of antibody. The antigens have been partially purified by proteolytic digestion and Sepharose 4B chromatography. Biochemical and physical characteristics of the synovial and bacterial antigens were found to be similar. The immunologically reactive site is consistent with a high molecular weight polysaccharide, Proprionibacterium polysaccharide (PPS).