SUMMARY: The intracerebral invasion in mice by two strains, a virulent strain and the strain 18–323 which is very virulent on intracerebral injection, was studied. The LD50 of these strains differed by about a thousand times and the average slopes of the regression lines from which these were calculated differed significantly, ( = 0·04 and = 0·01). It was found that the number of organisms/mouse brain in infected animals reached a maximum count which was slightly higher for the more virulent strain. The day on which the maximum count was reached depended both on the dose of organisms and the strain used, but occurred always before the seventh day after injection. Further, a slightly lower maximum count was reached as the dose of organisms was decreased. The passive protection by rabbit sera, described as antibacterial and antitoxic sera, against both these strains of pertussis organisms, was studied. It was shown that the protection of neither type of serum was due only to the presence of agglutinin antibodies.


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