Rabbit peritoneal exudate polymorphonuclear leukocytes (PMN's) aggregate in shaken suspensions and their adhesiveness, as judged by the extent of aggregation, can be modified by various agents. Vasoactive amines, nonsteroidal antiinflammatory drugs, autonomic agonists, cholinergic agents, and adenine nucleotides have only minor inhibitory effects. Cytochalasin B, ionophore A23187, bacterial endotoxin, and the phagocytosis of bacteria all markedly increase adhesiveness. Enhanced aggregation correlates positively with increased secretion of lysosomal enzymes, although the contents of the secretory granules are mild inhibitors of aggregation. The implications of the linkage between the process of secretion and the increased adhesiveness of the cells are discussed.