Three contrasting examples of bacterial host-parasite relations were studied with a view to recognising the compounds and processes responsible for the pathogenicity of the causal organisms. In the belief that only bacteria collected from only the specific disease-process in animals (i. e. An vivo) could be relied, on to have the complete armoury of virulence factors, the three bacterial species selected: Bacillus anthracis, Pasteurella Westin and the calla group, were all initially grown in vivo. An adequate supply of such organisms was obtained, and fractionation studies of the organisms and their products for substances responsible for virulence were made, possible by the use of biological assays which simulated conditions in the host. Once the important constituents of the bacteria grown in vivo had been recognised,it was often possible to alter the culture conditions in vitro to produce bacteria having the same constituents. The main findings were as follows- Anthrax virulent organisms. had-to have two factors (1) the capsule of polyglutamic acid which prevented opsonization and (2) the extracellular toxin, which was a mixture of three components, and poisoned the defence phagocytes: The massive bactersomia found in dying guinea pigs was not essential for death.