Eight patients suffering from severe rheumatoid arthritis were treated with high-dose human immunoglobulin. Doses of 500 mg/kg were administered intravenously in the morning on 4 consecutive days. Improvement of arthritic symptoms was indicated by a significant decrease of the Ritchie index, the number of swollen joints, and the duration of morning stiffness as well as an increase of grip strength. The improvement lasted up to 4 months. In 2 patients the disease activity increased again after 4 weeks. Investigations of laboratory parameters demonstrated a significant reduction of the acute-phase response. Parallel investigations of immunological parameters revealed a decrease of Il-6 levels in the sera. Therefore, we suggest a decreased activation status of the monocyte-macrophage system and T cells as a possible mode of action of high-dose immunoglobulin therapy in rheumatoid arthritis.