Hematologic abnormalities associated with penicillin compounds are uncommon, and neutropenia associated with ampicillin is reported even less frequently. Neutropenia developed in three pediatric patients after high-dose (150-400 mg/kg) ampicillin therapy over a period of 3 to 12 days. In all cases, the white blood cell and neutrophil counts returned towards normal within 4 to 11 days after discontinuation of the antibiotic. Bone marrow examination revealed a maturation arrest in one and slight shift to the left in the maturation of granulocytic cells in another. Other marrow components were normal. Red blood cells, reticulocytes, platelets, and hemoglobin did not show any abnormal alteration in any of the patients. Physicians administering ampicillin, particularly in high doses, should be alert to the possible development of neutropenia; however, all reported neutropenias have been reversible.