We administered Escherichia coli-derived recombinant human granulocyte-macrophage colony-stimulating factor to 61 patients with malignancy, 36 of whom had normal peripheral blood counts and 25 of whom had peripheral cytopenia due to underlying bone marrow disease, to compare the efficacy of two different routes of administration to stimulate the in vivo granulopoiesis: i.e., continuous i.v. infusion and s.c. injection. Three well-tolerated dose levels were investigated. Application of granulocyte-macrophage colony-stimulating factor resulted in dose-dependent increases in circulating neutrophils, eosinophils, and monocytes and an increase in bone marrow cellularity, irrespective of route of administration. In some patients, mild side effects, including bone pain, dyspnea, flu-like symptoms, and a decrease of platelet counts, were recorded, but they were less pronounced when the hormone was administered subcutaneously.