Ninety-three patients with a variety of joint diseases were studied for evidence of immune complexes in articular collagenous tissues. Frozen sections of freshly obtained biopsies of hyaline articular cartilage and menisci were stained with fluoresceinated monospecific antisera for evidence of human immunoglobulins (IgG, IgM,IgA) and the beta1c component of complement. The criterion for the presence of complexes was the staining of two or more immunoglobulins and beta1c in an identical location of sequentially cut sections. Of the 42 patients with rheumatoid arthritis (RA) 83% were positive by this criterion. In those with classic RA the incidence was 92%. Sixteen patients with fresh joint trauma or nonarthritic disease had negative findings. Among 26 patients with noninflammatory disease, 4 of 8 with polyarthritis whose features suggested primary degeneration, 1 of 11 patients with secondary degenerative arthritis, and a single case of synovial osteochondromatosis had positive findings. Among 9 patients with miscellaneous inflammatory arthritides, all of 3 with psoriatic arthritis were negative; however 2 of 6 with other inflammatory arthritides were positive. The findings in classic RA suggest that immune complexes are deposited in the articular collagenous tissues. The persistence of these complexes may play a significant role in the chronicity of the synovitis.