Granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) have recently been introduced in the treatment of chemotherapy-induced neutropenia. Effects of these CSFs on the cellular immune system were evaluated in 38 neutropenic gynecological cancer patients during chemotherapy. In addition to restoring the leukocyte count, GM-CSF--to a greater extent than G-CSF--also induced neopterin, a sensitive marker of macrophages activated by interferons. This effect was confirmed in vitro by investigating the effects of these CSFs on interferon-gamma-mediated pathways in THP-I human myelomonocytic cells. The results suggest activation of immune effector cells by GM-CSF.