The interleukins function as intercellular hormones, possessing the ability to alter the activity of a target cell population. Interleukin-4, secreted by activated T-cells, has shown antitumor activity in vitro against multiple myelomas, lymphoma, chronic myelomonocytic leukemia, and some solid tumors. Early promising clinical studies have shown the efficacy of IL-4 in decreasing the malignant lymphocyte count and in normalizing hematologic parameters in patients with CLL and in inducing transient clinical responses in patients with non-Hodgkin's lymphoma. Interleukin-6 possesses immunomodulating properties including enhancement of NK cell activity and induction of cytotoxic T-cell activity. IL-6 has shown antitumor activity in mice injected with weakly immunogenic syngeneic tumors and has been shown to inhibit in vitro human breast carcinoma and leukemia/lymphoma proliferation through a direct tumor inhibitory effect. Clinical studies investigating the antitumor activity of IL-6 are currently in phase II clinical trials. IL-6 and IL-11 have demonstrated thrombopoietic enhancing activity in primate models and early clinical trials. These agents have a potential application in ameliorating the thrombocytopenia associated with myeloablative chemotherapy. Yet to enter clinical trials, IL-12 has been shown to enhance the lytic activity of nonspecific NK/LAK cells and appears to be more efficient than IL-2 or IFN's in enhancing NK cytotoxicity. IL-12 has also been shown to enhance specific allogeneic human CTL responses and to induce the secretion of IFN-[gamma] from both resting and activated T and NK cells. In summary, these interleukins are now promising agents under investigation as effective treatment strategies in the oncologic setting.