The purpose of this study was to find out the causative organism of bovine hepatic abscess; the characteristics of this etiological agent, particularly its immunogenic capacity, and to determine the degree of immunity conferred by vaccination of laboratory animals. Sixty-seven liver abscesses were collected from cattle at slaughter, from 55 of which Sphaerophorus necrophorus was isolated in pure culture. In another 7, it was shown to be present in combination with other organisms. The morphological, cultural and biochemical characteristics of this organism have been studied. The abilities of various media to support growth, form filaments, reduce lag phase, and increase the stationary phase, have been studied. The effects upon growth of other contributory factors, such as dose of inoculum, age of the seed culture, pH of the medium, temperature of incubation have also been investigated. An attempt was made to immunize 13 rabbits and 7 guinea pigs by inoculating killed organisms. When challenged by the intravenous route with living culture, all the vaccinated animals resisted 7½ M.L.D., but succumbed to 15. In rabbits, intravenous inoculations of washed suspension of the organism produced a higher titre of agglutinins than the subcutaneous inoculations of heated whole culture. Rabbits with titres of 1:20,000 did not show any more immunity than those with 1:2500. The sera of cattle with hepatic abscess showed in general slightly higher agglutination titres than those free from abscess. However, this difference was on the whole slight. Hence, it is concluded that the agglutination test cannot be employed in the diagnosis of necrobacillosis. Judging by the cross-agglutinability, and despite some evidence of complex agglutinogenic structure, it was concluded that the various strains studied belong to one fairly homogeneous group. Two types of agglutinations, granular and floccular, were found to occur, depending on the average length and age of the organism used in the test. It was observed that the site of coherence which may correspond with the site of adsorption of the agglutinins is situated terminally or subterminally in both filaments and rods. / Science, Faculty of / Microbiology and Immunology, Department of / Graduate