Samples of peripheral blood of 27 patients with seropositive rheumatoid arthritis (RA) and 27 healthy age- and sex-matched controls were coded and studied for lymphocyte subpopulations. Monoclonal antibodies and indirect immunofluorescence were used to analyse subpopulations of purified T cells: OKT3 reacts selectively with all peripheral human T cells, OKT4 defines the helper/inducer subpopulation and OKT8 the suppressor/cytotoxic T cells. RA patients had a significantly lower relative lymphocyte count (p less than 0.001). whereas the percentage of all T cells was similar in patients and controls, RA patients had a significantly higher proportion of helper/inducer cells (62 +/- 1.3 vs. 56 +/- 1.0, p less than 0.005) and significantly decreased suppressors/cytotoxic cells (25 +/- 1.5 vs. 32 +/- 1.0; p less than 0.001). This resulted in an increased "immunoregulatory ratio" of helper to suppressor cells in RA patients (2.68 +/- 0.17) compared to their normal controls (1.83 +/- 0.10; p less than 0.001). The proportions of T cells expressing HLA-D-like antigens (defined by a monoclonal antibody to human Ia) was not different in patients and controls. These findings confirm conclusions derived from our previous study of T cell subsets defined by the expression of Fe-receptors (Tm and Tg), that in the peripheral blood of patients with RA, helper mechanisms predominate over suppressor mechanisms. This derangement of the immunoregulatory balance may have an important role in the pathogenesis of seropositive RA.