In 1940 Strean1 described the use of pertussis endotoxin as a cutaneous test to determine susceptibility to pertussis. In subsequent publications the same author2 described the preparation and properties of pertussis endotoxin and clinical studies in immunity to pertussis with the use of pertussis-skin-testing toxin and antiendotoxin. This work has been confirmed, in the main, in recent publications by Lapointe3 and Evans.4
Since the potency of the toxin produced by Haemophilus pertussis phase I seems to parallel the virulence of the organism,5 an attempt was made to enhance the virulence by serial passage in mice.
In these experiments six strains of H. pertussis phase I were used, ranging in age from 6 months to 6 years and cultivated on modified Bordet-Gengou medium for approximately forty to five hundred generations. The organisms grown on modified Bordet-Gengou medium responded to all the tests for a smooth phase