This study commenced with an investigation of the role immunological changes have in the production of the anaemia of schistosumiasis and of the role that complement-mediated mechanisms play in the production of that anaemia. The investigation of these mechanisms necessitated which developed in these animals was found to be mainly normochromic, progressive and of a haemoclytlc nature. A technique for the measurement of complement in the sera of the mice was developed using radioactive materials and employing an immuno-haemolytlc mechanism. Anaemia was found to be significantly more related to a fall in complement rather than to splenomegaly. The importance of complement in the production of anaemia led to the Investigation of its importance in other pathological processes involving rnembranes upon which complement could be deposited. Fluorescent techniques ware used for this part of the work. Nepthropathy caused by compliment deposits ln the renal glomeruli was particularly investigated. In view of the evidence existing stat, a similar mechanism might be Involved in trypanosomiasis, however, the study was extended to thls Infection. A study of complement in human infection with schistosomiasis was also carried out and in this study the complement was measured by radial immuno--diffusion test. A strong positive relationship between haemoglobin and complement was found in Infected patients but not in those with schistosomiasis but without anaemia, nor was complement found to be reduced in a series of controls. Treatment with Hycanthone in man was found to be followed six weeks later by a fall in in the complement level in the serum and in the serum content immunoglobulin M ( JgM ). This part of the work led to the deduction that there is an association Between complement and immunoglobulin M in attacking schistosonin worms. A further disease in which a complement-mediated mechanism was thought to be possible in sprue in in which damage to the intestined mucosa leading to malabsorption could be complement mediated. Some patients with sprue were therefore investigated. Similarities were detected, in so far compliment-mediated immune processes are concerned between patients with sprue and those with schistosomiasis.