Cells isolated from the joints of patients with rheumatoid arthritis (RA) exhibit functional immune abnormalities, such as diminished suppressor activity, depressed' response to mitogens, and enhanced immunoglobulin production. We sought to characterize the T lymphocyte subsets in the synovial fluid (SF) and peripheral blood (PB) of RA patients in an attempt to clarify the mechanism(s) responsible for these functional immune abnormalities. We used dual-immunofluorescence staining techniques with several combinations of monoclonal antibodies, including anti-4B4 and anti-2H4, which define, respectively, the helper inducer and suppressor inducer subsets of CD4+ (Leu3+ and T4+) cells. Mononuclear cells from normal PB (n = 9), RA PB (n = 6), and RA SF (n = 9) were analyzed, after staining, by flow cytometry. We observed a significant increase (P < 0.0002) in the number of cells bearing the helper inducer phenotype (CD4 + ,4B4 + ), and a significant decrease (P < 0.0002) in the number of cells bearing the suppressor inducer phenotype (CD4 + , 2H4+), in RA SF compared with the levels in PB from From the