Two types of strains of typhus virus are observed in Mexico: first the murine type which is obtained from wild rats in nature, from isolated endemic cases, and from cases during short epidemic outbreaks, and second, the epidemic type of strains which is obtained from long standing serious epidemics. Some of these epidemic strains correspond entirely to strains of historic Old World typhus. Other strains which in their experimental behavior are intermediate between these two types of strains were isolated from the same epidemic. A method is described by which these epidemic non-orchitic strains can be converted into murine strains regularly causing scrotal lesion in guinea pigs and a highly fatal disease in rats. The same results were obtained with an Old World strain of epidemic typhus. The method, which consists of daily blood injections into intraperitoneally inoculated rats, is based on the observation that Rickettsia prowazeki multiplies only within cells which come in constant or frequent contact with fresh blood. It is concluded from our experiments that there does not exist any real difference between the virus of historic Old World typhus and the murine New World typhus. Both are considered to be of murine origin. The murine strains represent the original form of the virus of typhus, whereas the epidemic strains are the result of a prolonged propagation in the cycle man-louse-man.