The effect of dermonecrotic toxin (DNT), fimbrial hemagglutinin (FHA), K-agglutinogen, lipopolysaccharide (LPS), and pertussigen from Bordetella pertussis on the production of IgE and IgGI antibodies to hen egg albumin (Ea) was investigated in C57BL/6 mice. The IgE antibody contents were determined by passive cutaneous anaphylaxis (PCA) in the skin of Lewis rats, while the IgG I antibody contents were determined by PCA reactions on the skin of mice using sera that had been heated for 3 hr at 56 C to destroy the IgE antibodies. Among the B. pertussis components tested, pertussigen was the most effective adjuvant for increasing the IgE and IgG I antibodies to Ea. LPS also moderately increased both types of antibodies, and FHA slightly increased the IgGI titers. When LPS was given 5 days before Ea, it suppressed both IgE and IgGI titers while FHA had only slight adjuvant action on both type of antibodies.When each of the components was tested for its ability to modify the adjuvant action of pertussigen, it was found that only DNT interfered significantly with the adjuvanticity ofpertussigen when given on the day of immunization with Ea. When the components were given 5 days before Ea, DNT produced significant suppression of only the IgGI response. LPS, FHA, and K-agglutinogen did not significantly affect the adjuvant action of pertussigen, B. pertussis cells excert an adjuvant effect on the production of antibodies in man and animals (32). This ability is shared by the lipopolysaccharides (LPS) of most gram-negative bacteria (26), but the adjuvant action of B. pertussis is due not entirely to LPS, but also to a protein called pertussigen by Munoz and coworkers (32) which has been referred to as a leukocytosis-promoting factor (LPF), leukocytosis-promoting factor-hemagglutinin (LPF-HA), histamine-sensitizing factor (HSF), and islet-activating protein (lAP) (6,34,45,51). Pertussigen has the unique property of enhancing the IgE class of immunoglobulins, an effect first observed by Mota (28) employing whole-cell pertussis vaccine. Many investigators have since confirmed the adjuvant action of pertussis vaccine and pertussigen (32). Munoz et al (34) found that as little as 0.1 ng of highly purified crystalline pertussigen increased production of IgE and IgG 1 antibodies to hen egg albumin (Ea).