Penicillin G sodium, 18 million units daily, was given intravenously to a 32-year-old man with bicuspid aortic valve and infective endocarditis. Three weeks later, he developed chills, fever and sore throat. Hemogram showed agranulocytosis: the white cell count showed 2100/mm^3 with 0% neutrophil. Penicillin was discontinued. Three days later, a bone marrow aspiration revealed myeloid hyperplasia with left-shifted granulopoiesis. The neutrophil counts returned to normal in 5-10 days. We believe that the case demonstrates the importance of close monitoring of the white blood cell count in patients with long-term, high dose penicillin therapy.