When nonimmune guinea pigs are inoculated intradermally (i.d.) with Rickettsia mooseri (R. typhi), the rickettsiae replicate at the site of inoculation, leading to the development of a grossly observable lesion. In contrast, guinea pigs which have recovered from R. mooseri infection are resistant to challenge and prevent both rickettsial growth and the formation of lesions. To study the mechanisms of this immunity, sera or splenic cells collected from nonimmune or immune guinea pigs were inoculated separetely into nonimmune recipients. Splenic cells collected from immune donors protected R. mooseri-naive recipients from i.d. challenge as measured by control of rickettsial growth and by prevention of development of lesions at i.d. sites of inoculation. In contrast, serum from immune and nonimmune doners failed to protect nonimmune recipients by either criterion.