G-CSF administration to normal donors results in granulocyte apheresis yields generally greater than those observed with other neutrophil mobilizing agents. In vitro, neutrophils cultured with G-CSF exhibit prolonged survival; however, the random migration of neutrophils exposed to this agent is inhibited. Although transfused neutrophils mobilized with agents other than G-CSF migrate to sites of inflammation or infection in vivo, this has yet to be demonstrated with infusion of G-CSF-mobilized neutrophils into neutropenic human subjects. Five neutropenic allogeneic bone marrow transplant (BMT) patients each received a fresh infusion of G-CSF-mobilized indium-labeled irradiated white blood cells (WBC) apheresed from HLA-matched normal donors on day +5 post-transplant. Localization of activity on delayed scintigraphic images of indium-labeled WBC scans to sites of tissue damage (oral/nasopharynx in two patients with mucositis and terminal ileum/cecum in one with diarrhea) occurred, and supports the hypothesis that G-CSF-mobilized HLA-matched donor neutrophils which have been irradiated are functional after infusion into neutropenic recipients.