(1) Two strains of tropical typhus have been isolated from wild rats trapped in endemic areas.
(2) Both were isolated and maintained in guinea-pigs.
(3) One could be isolated in rabbits by the intra-ocular inoculation of virus, and maintained thus indefinitely. The features of this infection appeared to be identical with corresponding infections obtained with the viruses of rural typhus and the tsutsugamushi disease of human origin. Repeated attempts to isolate the other rat strain in rabbits in the same manner failed.
(4) Rickettsia were found with ease and in abundance in infected material from guinea-pigs infected with either strain, and from rabbits infected with the one-strain.
(5) The sera of rabbits infected with the two rat strains gave positive Weil-Felix reactions of significant titre.
(6) Cross-immunity tests in rabbits between one rat strain and six strains of huinan origin of rural typhus and tsutsugamushi showed a cross-immunity to exist, complete in five strains and partial in the sixth strain.
(7) A concomitant infection with sodoku was present in the guinea-pigs of botlh strains ; although this may have modified the clinical signs, the infection by a typhus virus could be determined by four decisive criteria.
(8) The conclusion is drawn that the murine origin of the virus of the rural typhus-tsutsugamuslhi group of diseases is now firmly established.