We studied the in vitro expression and regulation of the CD23 and CD25 (Tac) surface antigens by peripheral blood lymphocytes (PBL) from severely burned patients (burn injuries ranging from 25% to 72% TBSA) in order to evaluate T- and B-lymphocyte activation processes after thermal trauma. The spontaneous and cytokine (IL-4, IL-2)-induced expression of CD23 which represents a B-cell activation marker was significantly reduced during the second to fifth week postburn when compared to healthy donors. In contrast, CD25, which is expressed on activated T cells, showed a marked increase both spontaneously, indicating an in vivo activation, and after stimulation with IL-2 or PHA. Concomitantly, T-cell proliferation induced by PHA or Con A was suppressed. However, the number of T and B cells remained unchanged. The data demonstrate the impairment of early events in the lymphocyte program in severely burned patients. The activation of B cells is downregulated, since they become refractory to external helper signals. In addition, T cells are highly activated but fail to proceed to proliferation in response to mitogenic stimuli.