Immunotherapy with interleukin-2 (IL-2) and lymphokine-activated killer (LAK) cells results in significant tumor regression in patients with advanced cancer. We have investigated the kinetics of circulating erythroid (BFU-E) and granulocytic-macrophage (CFU-GM) progenitors after IL-2 therapy in 11 cancer patients, mainly affected by metastatic melanoma and renal cell carcinoma. Administration of IL-2 from day 1 through day 5 constantly induced a dramatic decrease of the number of circulating BFU-E and CFU-GM, which then showed a striking rebound (up to values fourfold and sevenfold higher, respectively, than the pretherapy levels) on discontinuation of IL-2, ie, from day 5 through day 10. A similar kinetic pattern was observed during and after the second cycle of IL-2 administration. 3[H]-thymidine killing experiments showed that the cycling activity of the progenitors was virtually unmodified in the rebound phases. To explore the mechanism(s) underlying this kinetic pattern, we have analyzed the plasma concentration of several hematopoietic growth factors, including IL-1 beta, IL-3, IL-4, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), G-CSF, and erythropoietin (Ep). No modifications in the levels of IL-3, GM-CSF, or IL-1 beta were observed, whereas a pronounced increase of IL-6 and G-CSF concentration was monitored, starting at day 3 and peaking at day 5 of treatment (a parallel, but modest, increase of Ep level was also observed). The elevation of IL-6 and G-CSF concentration is directly correlated with and may, at least in part, underlie the subsequent rebound of circulating hematopoietic progenitors. Furthermore, the increase in IL-4 level observed at day 10 of therapy may mediate the eosinophilia gradually starting at this stage of treatment.