The effects of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) on neutrophil lactoferrin (LF) and transcobalamin (TC) 1 and 3 secretion were determined in vitro and during in vivo administration in humans. In whole blood, in vitro incubation with GM-CSF reproducibly produced a rise in plasma LF concentration (P less than 0.05) whereas in purified neutrophils the results were variable. Exposure of whole blood to GM-CSF also resulted in a significant rise in plasma TC 1 and 3 (190 +/- 60%, P less than 0.05). The response was dose dependent with maximal effect at GM-CSF concentrations of 10 ng/ml and above. rhGM-CSF was administered on seven occasions to six patients with malignant disease prior to chemotherapy. Plasma LF and unsaturated TC 1 and 3 levels rose significantly in each patient studied and the rise coincided with the initial neutropenia due to margination that occurs during infusions of rhGM-CSF. Patients receiving rhGM-CSF may therefore have hypofunctional neutrophils due to secondary granule depletion.