This collection of publications can be considered under three main headings: I. Life cycle and developmental studies on strigeoid trematodes, 2. A study of the host-parasite interface. 3. The reproductive biology of Schistosoma mansoni. The main theme throughout these papers is the study of the host-parasite relationship emphasising selected aspects of parasite biology, structure and histochemical characteristics which are significant to this relationship. The early studies on the strigeoid trematodes drew my attention to the very intimate attachment of these parasites to host tissue via the very specialised adhesive organ. This close physical relationship emphasised the importance of the host-parasite interface in the interaction between two genetically different organisms. This interface, through which all biologically important substances, of both host and parasite origin, must pass, can be regarded as comprising not only the external surfaces but also the lining of the gut, when present. Several examples of trematodes, cestodes and nematodes have been investigated on a multi-disciplined basis using light microscopy, transmission and stereoscan electron microscopy combined with histochemical techniques wherever appropriate. In the strigeoid trematodes in particular, considerable surface specialisation can be associated with the secretory and absorptive processes of the parasite. Similarly, in the cestode Moniezia, surface differentiation in relation to secretion was observed and in Taenia it was found that the phosphatase characteristics of the tegument varied along the length of the worm. In the nematodes, considerable contracts were revealed between the intestinal epithelium and the rest of the gut. This information has been useful in understanding the nature of the host-parasite interface and its function in the host-parasite relationship. In Schistosoma mansoni, a different aspect of the relationship is being studied. The accumulation of eggs laid by the female in host tissues is a stimulus for a delayed hypersensitivity response by the host. and this produces considerable damage to the host tissues. The functioning and co-ordination of the female reproductive system of this parasite is poorly understood and initial studies have been directed at elucidating basic structure and investigating the subcellular effects of known egg-laying inhibitors eg. schistosomicidal drugs on the reproductive system. These studies have produced interesting information on the differential susceptibility of the cells of the reproductive system to drug action, evidence of drug resistance by some cells and the relationship between cytosegresome formation and drug sequestration by others. It is hoped that these studies will provide a basis for the development of more specific inhibitors of egg production and so remove a major cause of host pathology. In the study of some aspects of the host-parasite relationship it is essential that the contact between both organisms remains undisturbed. This poses considerable technical problems in studying the more dynamic aspects of the relationship particularly when these occur at a sub-cellular level. Consequently, rather indirect methods as electron microscopy coupled to autoradiographic and histochemical techniques have to be used. Very recently new approach has become available which will permit the detection of biologically important elements in situ at an ultrastructural level. Introduced foreign elements can also be detected. The technique is the X-ray analysis of ultrathin sections in the transmission electron microscope. - Many problems remain to be overcome in the application of this technique, but some progress has been Made and its initial application has been to a study of drug distribution in Schistosoma mansoni. It will have also considerable value in the study of the host-parasite interface, providing a completely new dimension to the multidisciplined investigations.