Twenty-one patients with rheumatoid arthritis (RA) were investigated for various immunological parameters, both humoral and cellular. IgG concentration was 1673+/-266 mg/dl, IgM 259+/-108 mg/dl and IgA 302 +/-7 mg/dl. Enumeration of T lymphocytes in peripheral blood revealed a value of 66% with a B cell count of 10%. Additionally, IgG levels, in 5 selected patients, appeared to fall to normal levels in the course of treatament with D-penicillamine. The significance of these findings are discussed.