The contribution of humoral immunity against Bartonella henselae was evaluated by examining the in vitro bactericidal activity of sera and the ability of these microorganisms to activate complement and stimulate phagocytosis and an oxidative burst in polymorphonuclear leukocytes. The organism was killed by complement-mediated cytolysis. Complement activation preferentially proceeded by the alternative pathway. The presence of specific antibodies did not increase the serum bactericidal activity or complement activation. However, phagocytosis and the subsequent production of oxygen radicals, evaluated by flow cytometry, were significantly enhanced in the presence of bacteria previously opsonized with immune sera.