The effect of an extract containing the histamine-sensitizing factor (HSF) of Bordetella pertussis on the immune response of mice to ovalbumin was investigated with respect to dose of antigen and adjuvant. Of particular interest was the enhancement of reaginic antibody production. In comparison to the Al(OH)3 induced production of reaginic antibody where low doses of antigen and adjuvant yield high titers of reagin, the HSF extract demonstrated optimal adjuvant activity at high doses of both adjuvant and antigen. The reaginic antibody response was maximal usually by 2 to 3 weeks post-immunization and persisted for long periods of time. The hemagglutinating antibody response was maximal at 8 to 10 weeks post-immunization. The initial treatment of mice with HSF extract plus antigen resulted in the production of memory cells since a subsequent immunization with ovalbumin alone evoked a secondary reaginic response. These observations may have implications in clinical allergy since substances similar to the pertussis factor might be produced by other microbial organisms and these substances could modulate the immunologic response of individuals to common allergens.