Studies on human trichinosis have shown that eosinophilia is so constantly associated with trichinosis that its presence may serve as an important factor in the diagnosis of the disease. Considerable variation occurs in the degree of eosinophilia and the length of time it persists. There is a pronounced leucocytosis and eosinophilia in human infections. Cases with no eosinophilia but with a general leucocytosis are cited by Conner (1929) as an atypical form of trichinosis. Although the haematologic response of man to infections with Trichinella spiralis is quite well established, the same cannot be said of a number of laboratory animals that are used a great deal in experiments on this parasite. Maass (1933) made blood counts on trichina infected hogs. Of nine animals with normal blood pictures, two each were fed 100, 200, 1,000, and 2,000 encapsulated trichinae from muscle; the ninth pig served as a control. Eosinophiles increased gradually beginning a few days after infection; reached a maximum in 20 to 24 days (that is when the young trichinae penetrated the muscle) constituting 10, 12.5, 20, and 27 per cent, respectively; then fell rapidly, and gradually returned to normal within two or three days. Eosinophilia in the control animal varied irregularly from 0.5 to 4.5 per cent. At autopsy all the hogs that had been infected showed trichinae, particularly in the diaphragm, and also in the cheek, thoracic and the thigh muscles. The eosinophilia described by Maass was accompanied, in every case, by a leucocytosis in proportion to the number of worms ingested. In monkeys McCoy (1932) was unable to demonstrate a marked eosinophilia in his experimental work. Wantland (1937) studied the blood in normal and trichina infected white rabbits and found that variations occur even in normal animals, but in rabbits infected with 5,000 to 10,000 larvae a leucocytosis developed from the second week after infection. The animals that showed the severest symptoms also showed a greater eosinophilia; however he concluded that the percentage of eosinophiles was not an index of the severity of the infection. Van Someren (1938) in a series of differential counts on rats, both