Polymorphonuclear leukocytes from the rabbit are known to possess two types of granules - azurophils containing lysosomal hydrolases together with myeloperoxidase and other microbicidal agents, and specifics containing alkaline phosphatase, lysozyme and lactoferrin but lacking common lysosomal enzymes. The presently available data strongly suggest that a comparable situation is encountered in polymorphonuclear leukocytes of other species including man. The known leukocyte granule enzymes are listed together with their properties, and their possible functions are also described.