Interleukin-2 (IL-2) is a potent inducer of lymphokine-activated killer (LAK) activity against both NK-sensitive and NK-resistant tumor cell lines. IL-2 therapy has been associated with clinical responses, but these responses have occurred only with high, toxic doses of the cytokine. Since prothymosin alpha (ProT alpha) is able to enhance the spontaneous NK activity of cells from normal donors, we studied the effect of ProT alpha on LAK activity by IL-2. The lysis of K562 and Daudi cells by effector cells cultured with IL-2 was increased time dependently when cultures also contained ProT alpha. PBLC was separated by discontinuous density centrifugation in the LGL-enriched fractions. Within the CD16+ population, all effector cell precursors were CD2+, but the effector population after IL-2 or IL-2 + ProT alpha activation also contained CD16+CD2- cells; the CD2 molecule is thus indispensable for induction of LAK activity by IL-2 or IL-2 + ProT alpha but not for the action of activated LAK cells (or for the enhancing effect of ProT alpha). Within the CD3- CD16+ LGL population, 5 micrograms/ml ProT alpha plus 50 U/ml IL-2 was able to induce p70 IL-2R expression to a similar extent to 100 U/ml of IL-2. The use of ProT alpha to enhance the induction of LAK activity by IL-2 may be able to improve immunotherapy of cancer.