Patients with refractory carcinoma were treated with recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) by intravenous (IV) infusion. During the period of treatment, studies of polymorphonuclear leukocyte superoxide (O2-) release in response to formylmethionylleucylphenylalanine (fMLP) and phorbol myristate acetate (PMA) and studies of chemotaxis in response to fMLP and C5a were performed. We observed that patients receiving rhGM-CSF in vivo exhibited primed O2- release after stimulation both with fMLP and PMA. Chemotaxis, however, was not enhanced by the treatment. These data suggest that host defenses may be enhanced by this treatment and that rhGM-CSF may be a useful therapeutic adjunct in compromised patients.