Peripheral blood (PBL) and synovial fluid lymphocytes (SFL) from 18 patients with definite rheumatoid arthritis (RA) and from one patient with Reiter's disease (RD) were examined for their capacity to absorb quantitatively interleukin-2 (IL-2) from a standardized, lectin-free IL-2 source. For comparison normal ConA blasts and PBL from various inflammatory and noninflammatory diseases as well as from healthy control persons were studied. IL-2 activity was quantitated by measuring 3H-thymidine-2-deoxyriboside uptake in the IL-2 dependent murine T cell line CTL6. ConA blasts exhibited a high IL-2 absorption capacity and served as a positive control for calibrating the absorption assay. In a population of normal PBL at least 5-10% of ConA blasts were required to detect IL-2 absorption. Significant absorption was assumed if more than 50% of IL-2 activity was removed from 200 microliters of a lectin-free IL-2 standard following incubation with 5 X 10(6) lymphoid cells for 2 h at 4 degrees C; this criterion was fulfilled with 8 out of 20 SFL and 4 out of 14 PBL preparations from RA patients. As a rule SFL absorbed more IL-2 than PBL. Control PBL did not absorb significant quantities of IL-2. PBL from the RD patient apparently produced an IL-2 inhibitor during incubation with the IL-2 standard. IL-2 absorption by ConA blasts and SFL was fully inhibited by preincubation of the absorbing cells with monoclonal anti-TAC antibody, a reagent known to react with the human IL-2 receptor. The results are discussed in view of current concepts of antigen/mitogen induced T cell activation.