Recombinant human interleukin-2 (IL-2), administered to cancer patients by continuous intravenous (IV) infusion (3 x 10(6) U/m2/d), was found to induce the in vivo production of colony-stimulating factors (CSF). Plasma obtained from patients during IL-2 treatment stimulated in vitro colony formation of normal human bone marrow cells, depleted of mononuclear phagocytes and T lymphocytes. This colony-stimulating activity (CSA) was identified as IL-5, granulocyte-macrophage CSF (GM-CSF), and macrophage CSF (M-CSF), by the ability of specific antibodies against these factors to neutralize their effects. The presence of IL-2-induced GM-CSF and M-CSF was also demonstrated by specific radioimmunoassays. During IL-2 treatment, plasma also contained detectable levels of IL-6, which was measured in a bioassay. Using a cDNA-polymerase chain reaction (PCR) with specific primer sets for the various CSF, we showed that IL-2 treatment induced the expression of mRNA for M-CSF, GM-CSF, IL-3, and IL-5, but not for granulocyte CSF (G-CSF) in peripheral blood mononuclear cells, suggesting differential expression of CSF in vivo in response to IL-2. Furthermore, no negative regulators of hematopoiesis, such as interferon gamma (IFN-gamma) or tumor necrosis factor-alpha (TNF-alpha), were found in plasma. These data illustrate that in vivo administration of high-dose IL-2 may result in a stimulatory effect on hematopoiesis. The induction of detectable levels of IL-5 and GM-CSF in the circulation may explain the eosinophilia and neutrophilia observed in these patients.