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The blood protein vitronectin has been found to perform a variety of functions in the human body. Among these is its role in blood clot formation, the immune system, and tissue remodeling. By binding to the urokinase plasminogen activator receptor (uPAR), vitronectin facilitates cell adhesion and regulates tissue remodeling. This is a process that takes place in the formation of tumors and the progression of cancer cells. Vitronectin has also been found to play a key role in such pathogenic processes as invasion by Candida albicans yeast, which cause infections in immunocompromised hosts. This project aims to develop a better understanding of the molecular and cellular interactions of vitronectin binding to ligands. It aims to test the binding of vitronectin to the urokinase receptor and the effects of various factors on this binding. Among these factors are the interaction of vitronectin with Plasminogen Activator Inhibitor-1 (PAI-1) and the Extracellular Matrix (ECM) and the formation of multimeric vitronectin complexes. To determine the effects of these factors on vitronectin-urokinase receptor interaction, studies were conducted in vitro using isolated uPAR. These studies were performed by using the enzyme linked immuno-sorbent assays (ELISA) methods. The results of these experiments show a competition of PAI-1 with the urokinase receptor for binding to vitronectin. While these results agree with recent structural studies of the receptors that have been pursued, they contrast with previous observations of the effects of PAI-1 on the binding of vitronectin to another class of receptors, integrins. ELISA methods were also employed to develop appropriate conditions for the study of the binding of vitronectin to C. albicans cells. These assays showed increased binding to ECM with increased concentration. The aspects that were evaluated included determination appropriate cell growth conditions, assay blocking agents, and cell growth media.