Phlebotomus major v. chinensis is the most favorable sp. for the development of Leish-mania. The flagellates become attached to the lining of the mid-gut and grow forward until they reach the anterior part of the gut. Invasion of the pharynx usually takes 6 days, and under favorable conditions about 25% of the flies show a proboscis infection. P. sergenti is an equally favorable host for the early development, but the parasite remains confined to the broad posterior regions of the mid-gut, does not become attached to the lining of the gut, and never invades the proboscis. Infection depends on the presence of undigested food material and soon disappears when the gut is empty. Four out of 14 patients with kala azar gave positive results when sandflies were fed on them, but only about 5% of the flies became infected. Experiments with a large series of hamsters infected with Chinese kala azar show considerable variation (zero to practically 100%) in the proportion of sandflies showing development of flagellates after feeding on infected animals. In hamsters there is a marked correlation between the number of parasites in the skin and the proportion of flies becoming infected. No correlation was found between the presence of parasites in the circulating blood and in-fectivity to sandflies. A study of 5 human strains in hamsters showed the existence of varying degrees of virulence, ranging from a strain which was completely non-infective to sandflies, up to one highly infective both to hamsters and to sandflies. Both P. major v. chinensis and P. sergenti, infected with flagellates, gave negative results when fed on normal hamsters, and also when their contents were inoculated into the skin of hamsters. The intraperitoneal inoculation of the con-tents of 1 or more infected flies gave positive results in 7 out of 124 experiments with P. major, and 2 out of 41 with P. sergenti. There is evidence that Chinese kala azar may be congenitally transmitted, and also that a number of children recover from the disease without treatment.