tide in electrophoretic mobility with these antigens, slides were cut in small segments and eluted, and the eluate was tested for biological activity. The results showed that maximal histamine-sensitizing activity, leukocytosis-promoting activity, and adjuvant activity for reaginic antibody were present in fraction 6. Adjuvant activity for hemagglutinating antibody appeared to be rather dispersed. Since biological activities were present in regions in which precipitating antigens were detected, experiments were performed to determine if antisera could react with these activities. When pertussis extracts were treated with rabbit antiserum, in vitro antigen-antibody precipitates developed. After removal of these precipitates it was demonstrated that the supernates with reduced antigen content contained minimal biological activity compared with extracts treated with normal rabbit serum. Furthermore, in vivo neutralization of histaminesensitizing activity, leukocytosis-promoting activity, and adjuvant activity for reaginic antibody was achieved by passive immunization of mice with rabbit antiserum to HSF prior to sensitization. From these results it was concluded that HSF is a pertussis antigen. Evidence suggests that it is a lipoprotein of approximately 90,000 daltons in size. In addition to histamine-sensitizing activity it contains leukocytoJ. ALLERGY CLIN. IMMUNOL. APRIL 1979