Anti-IgG antibodies (anti-IgG) of the IgE class were studied in sera from patients with juvenile rheumatoid arthritis (JRA), rheumatoid arthritis (RA) and patients with Felty's syndrome (FS) by use of an indirect immunofluorescence technique. Forty-two per cent of 26 patients with JRA had IgE anti-IgG in serum all in low titers. Positive reactions prevailed in patients with multiple joint involvement. Sixty-three per cent of 30 patients with RA and 80% of 20 patients with FS had IgE anti-IgG, the titers found in FS patients being significantly higher. In JRA and FS patients the IgE anti-IgG titers were correlated to the titers of anti-IgG of the IgG class, and for FS patients also with the IgM and IgA classes of anti-IgG. In six of 10 patients with RA the synovial fluid samples from both knees contained IgE anti-IgG. In four of these patients the titers of IgE anti-IgG were higher than in the corresponding serum sample, pointing to a local production. After G-200 Sephadex chromatography IgE anti-IgG were demonstrated in the void volume indicating the presence of these autoantibodies in immune complexes. IgE anti-IgG may be involved in the pathogenesis of JRA and RA by eliciting Type I and III reactions.