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1. Extracts of the blood of monkeys (Macaca mulatta) infected with Plasmodium knowlesi contain histamine. A mean total concentration of 0.15 mug/ml was present in the circulating blood.2. No histamine was detected in the blood of healthy monkeys.3. Vasodilatation and increased vascular permeability was observed at the site of injection of the extract into the skin and brain of guinea-pigs.4. Infiltration of the dermal layer by leucocytes was observed after intradermal injection of the histamine extract. A similar response was obtained in the brain.5. The extract of the blood from the infected animal produced hypotension in rabbits when administered intravenously.6. The pathophysiological significance of histamine in malaria is discussed.

In spite of extensive studies of the pathogenic activities of Entamoeba histolytica upon animal hosts by many investigators, the factors that govern the pathogenesis of the parasite are still largely unknown. Attempts have been made to determine the factors concerned in the pathogenicity of certain strains of E. histolytica from the aspect of parasite itself and from the point of view of its interaction with the host. The enzyme content of the trophozoites of the amoebae examined has been studied in detail, both in the living organism and in its extracts. The total patterns of proteolytic enzymes and of certain other enzymes were determined to establish whether it was possible to define the invasiveness of the various strains of E. histolytica already established as pathogenic by other workers or in our laboratories. It was found impossible to distinguish clearly between the so-called "pathogenic" and "non-pathogenic" strains of E. histolytica by such means.