A highly potent extract of the histamine sensitizing factor (HSF) of Bordetella pertussis was isolated by extraction of bacterial cells with urea buffer and subsequent gel filtration. This preparation of HSF also contained leukocytosis-promoting activity and adjuvant activity for reaginic and hemagglutinating antibodiesl Digestion of this extract with pronase or trypsin partially destroyed histamine-sensitizing activity, leukocytosis-promoting activity, and adjuvant activity for reaginic antibody, but did not affect adjuvant activity for hemagglutinating antibody. Antisera to HSF was prepared by immunizing rabbits with either whole bacteria or partially purfied extract. These antisera contained several precipitating antibodies to Bordetella pertussis extract demonstrated by immunodiffusion and immunoelectrophoresis. Antisera added in vitro to Bordetella pertussis extracts or passively administered in vivo to mice, reduced or abolished all biologic activities except adjuvant activity for hemagglutinating antibody. These results suggest that HSF might be an antigenic component of Bordetella pertussis which also possesses leukocytosis-promoting activity and adjuvant activity for reaginic antibody.