During the Hongkong epidemic the great Japanese bacteriologist Kitasato, who formerly worked with Koch in Germany, discovered a bacillus in plague-stricken patients, and showed by experiments that these bacilli if injected into lower animals produced in them symptoms of plague. Yersin simultaneously discovered the same germs in connection with plague. According to our modern notion of the causation of the disease, these germs must be considered to be the specific poison which produces the symptoms of plague. The bacilli are found in the blood, in the buboes, and in all internal organs of the victim of the plague. They are short rods with rounded ends, with a clear space or band in the centre, readily stained by the aniline dyes and showing very little power of movement. The size of the plague bacillus varies, and bacilli of same character, but of less virulent nature, have been found in the soil of infected places. Some bacteriologists observed some development after death in the bacilli, this, if confirmed by observations at Bombay, will be highly interesting from a bacteriological point of view. If mice, rats, guinea-pigs and rabbits are inoculated with the plague bacillus, they soon become infected and die, and in their internal organs the same bacilli are found. They are also found in the soil and dust of houses where plague patients were kept, but not invariably so. Kitasato found the bacilli in the blood of patients convalescing from an attack of plague even three or four weeks after all symptoms have disappeared. It has been found that the bacillus dies after four days, during which it is kept at a dry heat, or at the temperature of 80°C. or 176°F. for half an hour, or at that of 100° C or 212°F. for a few minutes. Its resisting power to chemical disinfectants is feeble, dying in a 1 per cent. solution of carbolic acid or of lime water. It develops easily in many culture media at the ordinary temperature (from 18° to 22°C). An alkaline solution of Peptone 2 per cent., with from 1 to 2 per cent. of gelatine, is the best nutrient medium for its cultivation.