The in vivo effect of yeast-derived recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) was investigated in 29 patients with advanced malignancy in phase Ib trial. Patients were treated at six different dose levels (30-1000 micrograms/m2/day) with either daily intravenous bolus injection or 24 hours continuous infusion for 5 days or 2 weeks. Administration of rh GM-CSF resulted in a broad spectrum of dose-, route-, and schedule-dependent hematopoietic effects. Sustained infusion of rh GM-CSF elicited a maximum 17-fold average peak increase of the total white blood cell (WBC) count with mainly neutrophils, eosinophils, and monocytes accounting for this rise, and increases in bone marrow cellularity with a shift to immature myeloid elements. Elevation of lymphocytes, platelets and reticulocytes was not induced. Within one week after discontinuation of treatment the leukocytosis had disappeared. Adverse reactions encountered with rh GM-CSF seen in 65% of the patients studied were never life-threatening and always reversible. They included mild myalgias, facial flushing, low-grade fever, headache, bone discomfort, nausea, dyspnoea and transient decline of platelet counts. These results suggest that rh GM-CSF can be safely administered at the doses and schedules employed and that it can induce in vivo some of the biological effects reported in in vitro studies. Although no objective antitumour responses have been seen, the ability of rh GM-CSF to increase turnover and function of leukocytes in vivo may prevent neutropenia and infections, when GM-CSF is adjunctively added to cytotoxic cancer therapy.