The mode of action of methotrexate in the treatment of rheumatoid arthritis is still questionable. Although in vitro results suggest an immunosuppressive effect of methotrexate, several clinical studies have failed to confirm these effects in patients treated with oral low-dose methotrexate. With respect to the highly variable bioavailability of methotrexate, we investigated the effects of an intravenous administration of 15 mg methotrexate per week on peripheral blood lymphocyte subsets in eight patients with rheumatoid arthritis. Methotrexate after 12 weeks significantly (P < 0.01) reduced total peripheral blood lymphocytes and led to a pronounced redistribution of lymphocyte subsets with a preferred reductive effect on B-lymphocytes (P < 0.005) and T-lymphocytes (P < 0.05). Natural killer cells and killer cell-like T cells, on the other hand, were unaffected by the treatment. Our results suggest a cell-type specific effect of intravenously administered low-dose methotrexate on peripheral blood lymphocytes. This effect, in our opinion, may contribute to the mode of action of methotrexate as an immunosuppressive drug in the treatment of rheumatoid arthritis.