Infection is a frequent cause of death in patients receiving bone marrow transplants. Although lymphocyte dysfunction has been observed in a few such patients, no systematic study of neutrophil function has yet been reported. Neutrophil chemotaxis was evaluated by a 51Cr-radioassay after bone marrow transplantation in 34 patients with acute leukemia or aplastic anemia. The response to a chemotactic stimulus (C5a) was severely depressed (less than 35% of normal) in 18 patients, moderately depressed (35-65% of normal) in an additional 6, and normal in 10 subjects. The mean response in the absence of graft vs. host disease and antithymocyte globulin administration was 73.3+/-9.2% (SE) in contrast to 29.7+/-9.6% (P is less than 0.01) in patients with graft vs. host disease treated with antithymocyte golbulin. Both graft vs. host disease and antithymocyte globulin were implicated since the presence of either factor alone was associated with depressed chemotaxis (31.1+/-4.9% for graft vs. host disease, P is less than 0.01; 17.0+/-7.8% for antithymocyte globulin, P is less than 0.02). When normal neutrophils were incubated with antithymocyte globulin in vitro, their chemotactic response was markedly suppressed in the absence of a cytotoxic effect. Transplant patients with defective chemotaxis experienced significantly more infections than those with normal chemotaxis, and analysis of specific etiologic agents showed that this was predominantly related to bacterial pathogens. Chemotactic inhibitors were detected in the sera of seven patients and elevated IgE levels were found in nine subjects, eight of whom had graft vs. host disease. Generation of chemotactic activity by endotoxin activation of serum was reduced in five patients. The results demonstrate a severe defect in neutrophil chemotaxis in some bone marrow transplant patients and suggest that neutrophil dysfunction may predispose to infection in such patients.