BACKGROUND Interleukin-2 (IL-2)-based immunotherapy has been shown to effect clinical responses in 15-35% of patients with metastatic renal cell carcinoma or melanoma. Despite its clinical efficacy, many clinicians refrain from using IL-2 because of the associated toxicity. This toxicity is believed to be mediated by such secondary cytokines as IL-1, tumor necrosis factor (TNF), and interferon (IFN)-gamma, which are produced by the patient's IL-2-stimulated peripheral blood mononuclear cells (PBMCs). METHODS Human PBMCs were stimulated with 1 nM wild-type recombinant IL-2 (rIL-2) or IL-2 analogs (R38A or F42K) that preferentially bind to the intermediate affinity IL-2 receptor (IL-2R). PBMCs were activated for lymphokine-activated killer (LAK) activity in 4-h 51Cr-release assays, using Daudi target cells. Cytokine content in the culture supernatants was determined by enzyme-linked immunosorbent assay. RESULTS Both R38A and F42K were capable of generating substantial LAK activity. Maximal specific lysis was 54% for PBMCs activated by R38A and 52% for F42K-stimulated cells, in contrast to 64% for rIL-2. In addition, analog-stimulated PBMCs secreted 59% of the IL-1 beta, 25% of the TNF-alpha, and only 8% of the IFN-gamma produced in response to rIL-2 (all p < 0.01 compared with rIL-2-stimulated secretion; one-way ANOVA). CONCLUSIONS IL-2 analogs that preferentially bind the intermediate-affinity IL-2R retain the capacity to induce substantial LAK activity despite a greatly reduced secondary cytokine production. Therefore, such IL-2 analogs may provide an effective, yet less toxic means of cancer immunotherapy.