Gamma globulins were obtained from rabbits hyperimmunized with the filamentous hemagglutinin (FHA) or the leukocytosis-promoting factor hemagglutinin (LPF.HA) of Bordetella pertussis. Double immunodiffusion analyses showed the antibodies to be highly specific for the homologous antigen. Intraperitoneal injection of anti-FHA or anti-LPF.HA into 10-day-old ICR mice 30 min before aerosol challenge with B. pertussis protected the mice from disease as evidenced by survival and a normal rate of gain in body weight. Both anti-LPF.HA and anti-FHA enhanced the clearance of B. pertussis from the lung and prevented leukocytosis which normally occurs after aerosol infection of mice with B. pertussis. Anti-LPF.HA, but not anti-FHA, neutralized the leukocytosis-promoting and histamine-sensitizing activities of LPF.HA when incubated with the purified protein in vitro. Anti-FHA, but not anti-LPF.HA, prevented the attachment of B. pertussis to mammalian cells (HeLa and Vero) in culture.