The expression of CD14, Fc gamma receptor I (Fc gamma RI), Fc gamma RII, and HLA-DR on peripheral blood monocytes from patients with rheumatoid arthritis (RA) was studied to investigate their nature and their role in the pathogenesis of rheumatoid synovitis. Peripheral blood mononuclear cells obtained from 9 patients with active RA, 8 patients with RA in complete remission, and 14 healthy individuals, were stained with various monoclonal antibodies and analyzed on a fluorescence activated cell sorter. The expression of CD14 as well as Fc gamma RI and Fc gamma RII was upregulated on peripheral blood monocytes from patients with active RA, although the expression of HLA-DR was not increased. In addition, the expression of Fc gamma RI and Fc gamma RII on monocytes was still upregulated in patients with RA in complete remission, whereas the expression of CD14 on monocytes was normalized in these patients. These results indicate that peripheral blood monocytes in patients with active RA are already activated to express higher densities of CD14. In addition, our observation that CD14 density was increased on a subset of circulating blood monocytes in active RA, that HLA-DR was not significantly altered and that Fc gamma RI and Fc gamma RII were increased in both active and inactive RA is not compatible with the expected actions of interferon gamma. Finally, it is suggested that peripheral blood monocytes in patients with RA may have intrinsic abnormalities as evidenced by the enhanced expression of Fc gamma R, which is repeatedly observed regardless of the disease activity of RA.