The intradermal inoculation of Mexican typhus virus into immune guinea pigs produces a local reaction which is similar in its appearance to the lesion observed in the skin of normal animals submitted to the same treatment. The reaction in the immune animal appears earlier and fades sooner than the lesion in the normal guinea pig.

The inoculation of heat-killed or formalin-killed Rickettsiae produces no significant reactions at the site of the intradermal injection in typhus immune guinea pigs.

The virus, inoculated intradermally, has been recovered from the local lesion 72 hours after the injection into typhus immune guinea pigs.

Normal guinea pigs and persons without a history of typhus fever present a congestion and some swelling of the skin at the site of the intradermal injection of formalinized Mexican Rickettsiae. The reaction appears 24 hours after the inoculation and fades within 48 hours. Heating the formalinized Rickettsia suspensions at 70°C. for 30 minutes renders them inactive in normal men and guinea pigs.

From the experiments reported in this paper it seems that the reactions observed in typhus immune guinea pigs submitted to a second inoculation of typhus virus, belong to the group of reactions presented by tuberculous animals (Koch's phenomenon) and the accelerated takes shown by immune persons submitted to revaccination with vaccinia virus.

A heat labile substance has been demonstrated in the formalinized Rickettsia bodies, which produces a reaction in the skin of normal men and guinea pigs.

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