The effects of NSAIDs on IgM rheumatoid factor production in vitro and on serum rheumatoid factor concentration in vivo were investigated using indomethacin (5.0 micrograms/ml), carprofen (10 micrograms/ml). IgM rheumatoid factor was isolated from lymphocytes of patients suffering from rheumatoid arthritis. NSAIDs inhibited IgM rheumatoid factor production in vitro. Furthermore, serum IgM rheumatoid factor was reduced when NSAIDs were administered in vivo. It is thought that NSAIDs effectively remove suppressor T-cells from the tonic inhibitory action of PGE2. This previously unrecognized action of NSAIDs may be a factor in their efficacy in rheumatoid arthritis.