Immunohistochemical studies were performed in synovial tissues from 40 patients with rheumatoid arthritis (RA), 9 with juvenile rheumatoid arthritis (JRA), 7 with psoriatic arthritis, and 4 with various rheumatic diseases. Overall synthesis of IgG– and/or IgM–rheumatoid factor (RF) was found in all patients with seropositive RA and JRA, in 75% of patients with seronegative RA, and in 1 patient with psoriatic arthritis. Agglutinator producing cells were found in 77% of the samples from seropositive RA and in 44% and 56% from seronegative RA and JRA patients, respectively. The percentage of IgG plasma cells synthesizing one or more of the 5 types of agglutinators studied was approximately 10% of plasma cells synthesizing IgG–RF. Intercellular and intracellular immune complex deposits were also found in patients with seropositive and seronegative RA and JRA. These findings suggest that synthesis of agglutinators by synovial tissue plasma cells of RA and JRA patients is a distinct—but definitely less prominent—function than that of RF synthesis.