The formalin-killed suspensions of Mexican rickettsiae prepared according to the methods of Zinsser and Castaneda afforded a practical method for vaccination on a large scale. The vaccine has evident protective power against typhus infection, as proved by experiments in men and laboratory animals. Twelve persons who had not had a previous typhus infection were vaccinated and subsequently inoculated with large doses of typhus material obtained from infected guinea-pigs and none of them developed the disease. It was found necessary to inject at least 3 doses of vaccine in order to protect guinea-pigs against non-orchitic Mexican strains, whereas it was easy to immunize these animals with a single dose of vaccine against the orchitic typhus. The apparent immunological differences between orchitic and non-orchitic Mexican strains are the same as those observed between murine and European typhus.