To gain further evidence for the role of the Escherichia coli alpha-hemolysin in pathogenesis, its in vitro effects on human peripheral leukocyte function were studied. Leukocytes exposed to low doses of alpha-hemolysin responded with a marked chemiluminescence response, indicating activation of oxidative metabolism. This response was time and dose dependent. Pretreatment of leukocytes with doses of alpha-hemolysin at which nearly 80% of the cells survived decreased the ability of the cells to phagocytize bacteria and particles and to undergo chemotaxis. Premature activation of leukocytes and inhibition of phagocytosis and chemotaxis by alpha-hemolysin, if they occur in vivo, would greatly enhance the survival of an invading E. coli strain.