IT has been reported that mice injected with Bordetella pertussis vaccine become exceedingly sensitive to histamine¹. Maitland et al.² found that antibacterial rabbit sera against Bordetella pertussis contain antibodies to histamine sensitizing factor. However, it seems that no biological definition of histamine sensitizing factor has been established. We believe that it is a toxin different from the accepted pertussis toxin. The present communication deals with formation of the toxoid of histamine sensitizing factor in Bordetella pertussis with the use of formalin.