Almost nothing is known about how organisms and cells recognize foreignness, that is, how they distinguish between "self" and "nonself" materials. Molluscs and other invertebrates frequently respond to invading parasites by encapsulating the parasites with aggregations of blood cells called hemocytes, amebocytes, leucocytes, hemolymph cells, or phagocytes. In other cases, however, the host organism seems to lack any cellular defense or the cellular reaction is less marked. The proposed work will be devoted to searching for the basic mechanisms by which host blood cells do or do not respond to parasites, which is one of the most fundamental biological problems. Moreover, the work will go beyond the problem of the cellular basis of immunity because it will be concerned with the problem of cell adhesion, an important phenomenon in a variety of biological situations such as parasitism, morphogenesis, fertilization, and many pathological conditions, including cancer. The purpose of this work is to study the ultrastructure, cell surface carbohydrates, and enzyme activities of molluscan blood cells in snail-parasite interactions so as to reveal the factors and conditions which lead the hemocytes to recognize, or fail to recognize, the foreign nature of the parasite. The parasites to be studied are schistosome flatworms, the causative agents of schistosomiasis.