1. 1. The results of various studies by other workers on the spleen and parasite rates in Indians, Malays and Bantu are summarized. 2. 2. The results of our own observations on Khonds in India, and on Masai and Bantu in East Africa are also summarized and compared with the foregoing and with one another. 3. 3. It is pointed out that wide differences may be found in the malarial status of communities in the same country and, in the case of the Bantu, of the same race; while close similarities occur in different countries. 4. 4. The results of experimental infections in birds and monkeys are cited and interpreted in terms of the two states of allergy and immunity. The probable significance of these is illustrated by reference to the role of the two states in tuberculosis. 5. 5. It is suggested that the observed contrasts in the spleen and parasite rates in diverse communities may also be explained in terms of immunity and hypersensitivity. 6. 6. It is concluded that the immune status is dependent not on race but on the frequency of infection; and that the most accurate index of endemicity is the frequency of infections in the non-immune, that is the rate in early infancy. 7. 7. While a consideration of the Spleen Rate and Endemic Index may be of value in forming an estimate of endemicity and immunity when similar communities are being compared, these indices may be misleading when the epidemiological surroundings are dissimilar. 8. 8. A consideration of the variation with age in the spleen and parasite rates throws more light on endemicity and immunity and on such variation a rough classification may be made into immune and sub-immune communities. 9. 9. But it is suggested that by far the most sensitive index at present available for field studies is the average parasite count, and its variation with age. c 1937.