The immunology , immunochemistry and cellular biology of two parasites that cause considerable disease in the United States are studied. Giardia lamblia is the most common disease-causing parasite in the United States. Variant specific surface proteins( VSPs) cover the surface of Giardia and undergo surface antigenic variation where one VSP is exchanged for another antigenically distinct VSP. Another protein important for Giardia is a protease, ESCP that is important in allowing the Giardia to form cysts, the infectious phase of the organism that can survive outside the body. This protein resides in the membrane of an organelle peculiar to Giardia, the peripheral vesicle (PV) that has many features in common with the lysosome of mammals. Both of these proteins were studied to determine how Giardia targets secreted molecules to their destination in the cell, in this case the cell surface for VSPs and the PV for ESCP. A tyrosine based motif consisting of 4 amino acids present in the cytosolic tail at the end of ESCP, YRPI, known to be important in targeting mammalian proteins to the lysosomal membrane was found to target the protein to the lysosomal membrane in Giardia as well. In contrast the location of the VSP to the surface was unaltered after mutation of its conserved cytosolic tail but located to the lysosome with the addition of the YRPI motif. These and other studies indicated that the secretory system of Giardia has many similarities to that of higher organisms. Microsporidia are intracellular eukaryotes that infect many animals and cause opportunistic infections in AIDS patients. The disease is transmitted via environmentally resistant spores. Cells are infected via a polar tube that infects cells by penetrating the host membrane and injecting its infectious sporoplasm into the host. We reasoned that in order for spores to be able to infect cells efficiently, they would need to be anchored to the cell. This implies a specific attachment mechanism to cells. Studies found that spores specifically attached to cells by means of the matrix surrounding mammalian cells. Spore attachment to a number of epithelial cell lines was inhibited from 73-89% by heparin and chondroitin sulfate A and B, 66% by the tripeptide arginine-glycine-aspartic acid and by antibodies to beta integrins. MnCl, known to "activate integrins" led to 750% increase in adherence. These studies show that there are specific spore attachment mechanisms to mammalian epithelial cells which permit spores to easily and effectively infect cells. This also provides a rational to inhibit infection by microsporidia.