This project is concerned with analysis of the diversity of medically important parasitic protozoa and its implications to the epidemiology, course, and diagnosis of disease, the development of atomic force microscopy (AFM) for the study of infectious diseases, the development of high resolution, low-light-level video microscopy for the study of the interaction of parasitic protozoa with host cells, and analyses of host-parasite interactions at cellular and subcellular levels. We are in the process of developing protocols for the preparation, observation, interpretation, and analysis of biological material by AFM. We have demonstrated heretofore unobserved differences in topography between developmental forms of Trypanosoma cruzi, the causative agent of Chagas' disease in man. It is possible that these differences in topographic features could be involved in the ability of the trypomastigote stage to infect vertebrate cells. We have shown that tachyzoites of Toxoplasma gondii attach to but are incapable of being interiorized by erythrocytes. However, the degree of interaction with the erythrocyte may depend upon the animal species from which the erythrocytes were derived.