Granulocyte alloantibodies and autoantibodies have a key role in the pathophysiology of several clinical problems. These include febrile transfusion reactions, severe pulmonary reactions to transfusion, isoimmune neonatal neutropenia, failure of effective granulocyte transfusion, autoimmune neutropenia, drug-induced neutropenia, and neutropenias secondary to many other diseases. Although many techniques are available for detecting granulocyte antibodies, the optimal in-vitro tests for predicting the antibodies' clinical effects are not established. Use of indium-111-labeled granulocytes may provide valuable information regarding the in-vivo effects of different granulocyte antibodies. Granulocyte transfusions continue to be used for a limited number of severely infected neutropenic patients who do not respond to antibiotic therapy.