The human parasite, Schistosoma mansoni, causes a disease of major health importance in many regions of the world. During the life cycle of this organism, penetration of host tissues occurs at almost every stage. This project is aimed at studying the mechanisms by which the eggs of Schistosoma mansoni penetrate through host tissues. The penetration process is probably mediated by the elaboration of proteolytic enzymes. An acidic thiol proteinase has been isolated from schistosome eggs and has been partially characterized. Further studies to determine the physical properties of this enzyme, its enzymic specificity and its role in penetration are in progress. In addition, this enzyme will be compared with proteinases previously isolated from the cercarial and adult stages of this organism. Various antigens, elaborated by schistosome eggs, appear to elicit the host's immune response, which is the principal cause of pathology of schistosomiasis. The relationship of these proteinases to the immune response is currently under investigation.