Open Access

1. In investigating a spontaneous epidemic disease of rabbits, a micro-organism was isolated in pure cultures which reproduced the characteristic lesions of the natural disease. 2. The bacteriological characters of this bacillus are described and the impossibility of identifying it with previously recorded organisms justifies its being considered a new species. The name Bacterium monocytogenes is proposed. 3. Animal passage raised "virulence" when the doses were well chosen, and increased virulence accentuated the production of necrotic lesions. Overwhelming doses of culture resulted in lowering of "virulence" by animal passage. 4. Bacterium monocytogenes, in doses less than the M.L.D., produced in the circulating blood of rabbits an extreme monocytosis. The responses of the other white cells. were either transient or inconstant. 5. Repeated doses of the organism became progressively less effective as stimuli to large mononuclear production. 6. The cell content of the thoracic duct did not reflect the high degree of monocytosis in the circulating blood. 7. On intrapleural injection of peptone broth and B. coli, the cells of the resultant exudate were primarily polymorphonuclears, even though the circulating blood showed a high monocytosis. With intrapleural injection of B. monocytogenes, when the blood stream was rich in large mononuclears, a pleural exudate containing 30 per cent of these cells was obtained. 8. Phagocytosis experiments in vitro showed that the large mononuclears, while they phagocyted B. coli indifferently, took up B. monocytogenes with an avidity in all respects equal to that of the polymorphonuclear neutrophiles.