Convalescent serum samples were examined for the ability to promote phagocytosis of Bordetella pertussis by human neutrophils. One sample promoted phagocytosis, and 11 of the 51 samples caused a statistically significant reduction in phagocytosis, compared with that of bacteria not incubated with serum. Phagocytosis was influenced by interactions between antibodies that promoted phagocytosis and antibodies that inhibited phagocytosis. Adenylate cyclase toxin (ACT) has been shown to block phagocytosis by neutrophils. Antibodies to ACT were removed from the sample that promoted phagocytosis, by incubation with ACT-coated paramagnetic beads, and the depleted serum no longer enhanced phagocytosis. The adhesin filamentous hemagglutinin (FHA) has been shown to mediate attachment of B. pertussis to neutrophils in a way that promotes phagocytosis. Depletion of antibodies to FHA from samples that blocked phagocytosis improved phagocytosis, compared with the no-antibody control. These results suggest that antibodies to ACT can promote phagocytosis, whereas antibodies to FHA can counteract beneficial opsonins.