(1) Mice can be readily infected with many strains of B. tuberculosis, both of human and bovine type, by intraperitoneal inoculation with a dose of 0·75 milligramme of moist culture. But, with a fairly virulent culture of bovine type, doses of 0·005 milligramme produced infections of almost the same duration as the larger doses.

(2) The investigation of seven cultures of human and six of bovine type showed that in general the animals infected with the bovine strains survived. for a shorter time than those infected with the human strains. Also, several human cultures failed to produce infection, whereas no failure occurred with the bovine cultures. But no constant difference was found between the duration of infections with the two types of bacilli. Thus rapidly fatal infections were produced by certain cultures of both types.

(3) The period of survival of infected animals depends partly on the particular culture used for inoculation, and partly on the individual resistance of the animal and, in the case of nearly full-grown mice, is independent of their weight at the time of inoculation.

(4) The effect of repeated passage of tuberculous material from mouse to mouse without interposing cultivation of the bacilli on artificial media was investigated with eight strains, the series of passages numbering from three to eight. Increase of virulence was repeatedly observed, but such increase was never maintained in subsequent passages. The evidence indicates that the individual host plays an important part in modifying the virulence of the organisms for other members of the same species.

(5) As regards maintenance of virulence, comparatively constant results were obtained with different subcultures of the same strain. But repeated tests over a period exceeding five years have given indications that the virulence of a strain may become altered spontaneously from time to time.