BACKGROUND In adults--but not neonates--neutropenia has been reported to complicate treatment with intravenous immunoglobulin, but the mechanism is unknown. OBJECTIVE To describe for the first time the case of a newborn infant who, after intravenous immunoglobulin, demonstrated serum antineutrophil antibodies and neutropenia. METHODS The 1,425-g, 36-week-gestation boy was healthy except for intrauterine growth retardation. Intravenous immunoglobulin (1g/dose x 3) was administered to treat alloimmune thrombocytopenia. Neutrophil-specific antibodies were detected by a granulocyte immunofluorescence assay. RESULTS After the intravenous immunoglobulin, the platelet count normalized but the neutrophil count declined to 450/mm3. Neutrophil-specific antibodies were detected in the serum of the infant but not in the maternal serum. Furthermore, cross-matching revealed that the maternal serum did not react with the infant's granulocytes. Two of three random lots of intravenous immunoglobulin contained detectible anti-neutrophil antibodies. CONCLUSIONS After intravenous immunoglobulin, the infant's serum contained one or more anti-neutrophil antibodies that were not maternal in origin. We speculate that the neutropenia resulted from the administration of intravenous immunoglobulin containing antineutrophil antibodies.