The α-toxin (hemolysin) of Staphylococcus aureus is known to be an important determinant of pathogenicity although its precise role in the process of infection is not understood. In this study, the interaction of α-toxin with the human complement system was evaluated in terms of its effect on the opsonic activity of serum for S. aureus. Phagocytosis by human polymorphonuclear leukocytes was studied by measuring the uptake of preopsonized radiolabeled bacteria. It was found that α-toxin-treated serum had reduced opsonic activity and that this change was associated with complement consumption via the classical pathway. Levels of C3 to C9 were reduced in proportion to the amount of toxin added to the reaction mixture; levels of C2 were markedly reduced but those of factors B and D of the alternative pathway were unaltered in the presence of α-toxin. Heat-inactivated toxin, which had no hemolytic activity, also interacted with the complement system but with a significantly reduced effect. In addition, α-toxin behaved as a chemotaxinogen for polymorphonuclear leukocytes: human serum was activated by the toxin. These studies demonstrate that the interaction of staphylococcus α-toxin with human serum affects two important aspects of the host response to the staphylococcus.