In this thesis the aetiology and significance of eosinophilia in general has beer. discussed. A series of twenty-five cases is reported fully where an increase in the blood eosinophil: count was present in all. Observations on all the cases have been made in the commentaries on each and a discussion of the findings is finally conducted. The significance of eosinophilia in the native of South Africa has an important bearing on the public health eradication of excremental disease and prevention of pulmonary tuberculosis. In the solution of these problems three major facts are evident from this study. They are:- 1. The native who is responsible through his own peculiar social conditions, for the dissemination of excremental disease seldom complains of alimentary tracts disorders when seeking. medical attention. If a particular symptom be present in his case it usually is attributed to the respiratory system. 2. The sick African native with pulmonary symptoms should be closely studied with a view to the exact aetiology of his pulmonary syndrome. This was shown in the case series to be almost invariably due to a parasitic alimentary pathology; which when eradicated spontaneously cleared the pulmonary picture. The pulmonary signs seen radiologically simulated tuberculosis in several of the cases studied and unless the medical attendant is on the watch for a parasitic cause the man may inadvertently be diagnosed as tuberculous and suffer hardship. 3. In many instances-parasites were only isolated after repeated and again repeated examinations of the stool. It is unfortunately too easy a practice to treat the patient symptomatically, search once or even three times in the stools for parasites, and discharge him after a short rest. This attitude will not help to clear the country of infestation. If an eosinophil count were to be taken of every medical admission to hospital whatever the symptomatology, and when found positive the cause searched for and eradicated much saving of pulmonary syndrome and other symptom complex therapy would be effected and the interests of public health in South Africa advanced.