Detailed morphologic and enzyme cytochemical analysis was carried out by electron microscopy on granules of mature granulocytes obtained from the circulating chicken blood. Heterophils possessed three types of granules: large, rod-shaped, dense (Type I); medium sized, oval, light (Type II); and small-core (Type III). Acid phosphatase activity was present in Type I granules, but peroxidase and alkaline phosphatase were not demonstrable. The cytochemical nature of Types II and III granules remains unknown. Eosinophils contained only one type of granule, which was circular and had electron-opague contents. Both peroxidase and acid phosphatase, but not alkaline phosphatase, were present, indicating that the granules are lysosomes like the granules of mammalian eosinophils. Basophils possessed two types of granules, the characteristic large basophilic granules (Type I) and small dense granules (Type II). Acid phosphatase activity was found in only a small proportion of Type I granules: peroxidase and alkaline phosphatase were not demonstrable.