Biological response modifiers such as interleukin 2 (IL2) may be most effective in the setting of minimal residual disease. In a phase I-II clinical trial, IL2 was administered to 10 patients in remission of acute myeloid leukaemia and three with multiple myeloma 1-4 weeks after treatment with ablative chemotherapy or chemotherapy and autologous bone marrow transplantation. The aim was to assess the capacity of these patients to tolerate IL2 after intensive therapy and to determine whether regenerating lymphocytes were capable of responding to IL2 with the generation of anti-leukaemic effector cells. Toxicity was severe in two patients treated with escalating doses of IL2 and 19 subsequent infusions administered to 11 patients on a fixed dose schedule for periods of 3-5 days were well tolerated. Major toxicity was confined to hypotension (two courses) which responded rapidly to treatment cessation. No patients required intensive care unit support. IL2 infusions produced no significant adverse effects on marrow regeneration; while there were transient falls in platelet counts there were no episodes of clinical bleeding and neutrophil counts increased from a mean of 1.1 pre-infusion to 2.5 x 10(9)l-1 during the infusion (P = 0.004). A significant biochemical abnormality was hypokalaemia which responded rapidly to correction. Cells with activity against leukaemic progenitor cells appeared in peripheral blood within 48 h of beginning treatment. We conclude that IL2 may be used in minimal residual haematological malignancy, and by producing anti-neoplastic effector cells has the potential, as yet unproven, to prolong disease-free survival of patients entering remission.