A cell-free extract of human diploid B lymphocytes (Wil2) in continuous culture was used as the source of antigen in immunodiffusion to detect a precipitating antibody referred to as rheumatoid arthritis precipitin (RAP) in sera of patients with rheumatoid arthritis (RA). The prevalence of RAP was determined in various arthritides and other connective tissue diseases. It was found in 67% of patients with seropositive RA and in 62% of patients with Sjögren's syndrome associated with RA. But its frequency was lower in many other connective tissue diseases, including seronegative RA and Sjögren's syndrome without associated RA. Matching sera and synovial fluids from patients with seropositive RA were also studied. Differences in serum and synovial fluid RAP titers were demonstrated, but generally when RAP was present in serum, it was also present in synovial fluid and vice versa. Indirect immunofluorescence (IF) with Wil2 cells as substrate was used to demonstrate RAP and to determine morphologically the nature of the reactive cellular antigen. When cells were treated with commonly used fixatives, little or no staining with RAP positive sera was observed. This outcome was shown to be the result of the extreme solubility of the cellular antigen reactive with RAP. The antigen was retained best in a reactive form in Wil2 cells after fixation of cells with dry heat at 37 degrees C for 30 minutes, and it was demonstrated by IF staining as small round granules distributed in the nucleus and cytoplasm. IF was also employed to demonstrate that RAP is immunoglobulin belonging to IgG and IgM classes and not to IgA or IgD.