Monocytes derived from peripheral blood of patients with rheumatoid arthritis (RA) had a marked defect in their chemotactic and phagocytic activity. The chemotactic defect resides at a cellular level, while the phagocytic defect is serum-dependent. Both defects could be of importance in the pathogenicity of RA and the susceptibility to infection such patients show. Finally, monocytes were found to have a negative influence over spontaneous cytotoxicity mediated by T lymphocytes.