We have studied the cytotoxic profile and distribution of lymphocyte subsets of patients with acute myelogenous leukemia in second remission, after continuous infusion with recombinant interleukin‐2 (IL‐2). The patients received repetitive cycles of 1–1.25 × 10⁶ U/m²/day of IL‐2, given as 4 days continuous intravenous infusion followed by a 3‐day treatment‐free interval for the first 4 weeks. Patients receiving >4 cycles were treated with the same dose of IL‐2 continuously for 4 days, followed by a 10‐day treatment‐free interval. These studies showed that IL‐2 treatment resulted in the generation of peripheral blood cytotoxic activity against both NK‐susceptible, K‐562, and NK‐resistant Daudi cell lines. In most patients, enhancement of lytic activity increased with the number of IL‐2 infusions. The cytotoxicity in some patients increased as much as 700‐fold and 830‐fold against K‐562 and Daudi cells, respectively. It is of importance that oncolytic activity was also induced in bone marrow compartment (up to 182‐fold against K‐562). Some decline in cytotoxicity was observed within 14 days after initiation of IL‐2 infusion in peripheral blood, but high levels of lytic activity persisted at this time in bone marrow. It is of interest to note that the cytotoxicity of in vivo IL‐2 primed lymphocytes was further potentiated by IL‐2 in vitro. Importantly, the cytotoxic cells induced in vitro displayed lytic activity against fresh leukemic blasts. Phenotypic analysis demonstrated that CD3⁻, CD56⁺ NK cells were significantly increased by in vivo IL‐2 treatment (34 to 47‐fold in absolute numbers), while CD3⁺, CD56⁺ T‐cell subset remained low. Characterization of cytotoxic cells using the complement‐dependent assay and monoclonal antibodies indicated that both the in vivo‐induced and ex vivo‐potentiated lytic function was mediated by CD3⁻, CD56⁺, CD16^+/– NK cells.