Cancer patients undergoing interleukin (IL)-2-based immunotherapy frequently experience dose-limiting side effects believed to be caused by the actions of such cytokines as IL-1β, tumor necrosis factor (TNF)-α and -β, and interferon-γ (IFN-γ). Human peripheral blood mononuclear cells (PBMC) or monocyte-depleted peripheral blood lymphocytes were stimulated for up to 7 days by either of 2 IL-2 analogues (R38A or F42K) that bind to the intermediate-affinity IL-2βγ receptor but have reduced abilities to bind the high-affinity IL-2 receptor. We previously reported that these IL-2 analogues retain the ability to generate lymphokine-activated killing by PBMC. In this study, we analyzed the cytokine content of supernatants from stimulated PBMC and peripheral blood lymphocyte cultures by enzyme-linked immunosorbent assay. The secretions of IL-1β, TNF-α, and -β, and IFN-γ induced by either R38A or F42K were markedly reduced compared with secretions produced in response to recombinant wild-type IL-2. In 4 experiments, secretion was reduced an average of 39% for IL-1β, 57% for TNF-α, 83% for TNF-β, and 86% for IFN-γ. Polymerase chain reaction analysis of recombinant wild-type IL-2 or analogue-stimulated PBMC did not reveal the presence of IL-2 mRNA; thus, differential production of endogenous IL-2 could not account for these findings. These data suggest the interaction of IL-2 and the high-affinity IL-2 receptor on human PBMC or peripheral blood lymphocyte is required for maximal secretion of IL-1β, TNF-α, TNF-β, and IFN-γ. Because such cytokines are believed to mediate the toxicity seen with IL-2-based immunotherapies, IL-2 analogues with reduced binding to the high affinity IL-2 receptor may prove to be an effective and less toxic means of cancer treatment.