Patients with intractable rheumatoid arthritis (RA) were treated with total lymphoid irradiation (TLI, 2000 rads). We previously reported long-lasting clinical improvement associated with marked suppression of in vitro lymphocyte function in this group. In an attempt to better understand the mechanism of immunosuppression and clinical changes observed after TLI, we studied in greater detail changes in peripheral blood T-cell subsets identified by monoclonal antibodies. Before TLI, RA patients had a higher percentage of Leu-3 (helper subset) cells and a lower percentage of Leu-2 (suppressor/cytotoxic subset) cells than normals. Immediately after TLI, the absolute numbers of both Leu-2 and Leu-3 cells were reduced by at least 90%. Within 6-12 weeks, the number of Leu-2 cells returned to the pretreatment levels, but the levels of Leu-3 cells remained depressed for months thereafter. The lack of repopulation of Leu-3 cells resulted in a marked increase in the ratio of Leu-2 to Leu-3 cells as compared to pretreatment values (1.73 +/- 0.23 vs 0.39 +/- 0.06), and in a decrease in the percentage and absolute number of total T (Leu-1 and Leu-4) cells. The failure of Leu-3 cells (which mediate predominantly helper/inducer functions) to repopulate the peripheral blood may contribute to the prolonged clinical immunosuppression observed after TLI. Similar changes in T-cell subsets were not observed in RA patients given remittive drugs or low doses (200 rads) of radiotherapy. Thus, TLI differs from other treatment modalities with regard to its prolonged selective effect on the Leu-3 subset.