The structure of the specific granules of eosinophil leukocytes has been studied by electron microscopy in sections of tissues, buffy coats, and sediments of peritoneal washings of rats, mice, guinea pigs, and men. The core of eosinophil granules is a crystal which has a cubic lattice with a repeat of approximately 30 A in rodents and approximately 40 A in man. The chemical composition of the core is discussed in connection with recent cell fractionation studies, and the hypothesis that the core is a crystal of peroxidase is considered.