Twelve patients with acute myeloid leukaemia (AML) with evidence of resistant disease were treated with recombinant interleukin 2 (rIL2) given intravenously by continuous infusion. No objective response to rIL2 alone was documented in the seven patients with advanced disease (20-90% resistant blasts in the marrow), except for a partial response to rIL2 plus chemotherapy in one. Of the five patients with limited disease (8-15% marrow blasts), three obtained a complete disappearance of the blasts following two to four 5d courses of rIL2 alone. One patient persists in fourth complete remission (CR) 30 months later, another obtained a third CR for 4 months, and the last remained in third CR for 9 months before relapsing. This latter patient achieved a fourth CR with low-dose cytarabine. The remissions have been maintained with low-dose monthly courses of rIL2 given on an out-patient basis. Two AML did not respond to rIL2 alone; one, however, obtained a fourth CR with chemotherapy and rIL2. Administration of rIL2 was accompanied by organomegaly and leucocytosis, with a frequent lymphocytosis and increase in eosinophils and large granular lymphocytes, both in the blood and in the marrow. Side effects, though often severe, were controllable using a daily dose escalating protocol and never required intensive care treatment. The results of this pilot study indicate that treatment of AML patients with rIL2 is feasible and may result in the disappearance of chemotherapy-resistant blasts in patients with limited but detectable disease. Further controlled trials in AML in CR appear warranted.