The neutrophil function of seven patients receiving allogeneic bone marrow transplantion was studied. Five of the patients had been transplanted for aplastic anaemia and two for acute leukaemia. Determinations were made of neutrophil phagocytosis, chemotaxis, random migration, and microbicidal activity for Candida albicans and Staphylococcus aureus. One patient showed a decreased ability to kill C. albicans at a time when she had active pneumonia due to Pneumocystis carinii. The remainder of the studies showed normal neutrophil functions. No differences were observed in the patients who had graft versus host disease [GvH] from those without GvH. These studies suggest that defects in phagocytic neutrophil function do not contribute significantly to the impaired host defenses in recipients of bone marrow transplantation.