A member of the mammalian peroxidase family, lactoperoxidase (LPO) serves as the first line of defense in a variety of secretory fluids including tears, saliva, and cervical fluid. While lactoperoxidase is structurally capable of binding a variety of small molecules to its active site, only thiocyanate (SCN-) interacts with LPO while in the presence of hydrogen peroxide (H2O2) to form a biocidal agent. Recently, LPO was identified as contributing antimicrobial activity against bacteria such as staphylococci and E. coli while present in mammalian respiratory secretions. In cystic fibrosis (CF), a systematic disease characterized by incessant lung infections, inflammation, and increased mucin content, this antimicrobial system has been shown to dysfunction. This work focuses on understanding the structural role that solvent plays in lactoperoxidase's preference for its ligands and ultimately how this preference affects its functionality. The specific aims are as follows: 1) Use various bound ligands to probe the active site heme environment of LPO; 2) Examine the role of distal arginine in ordering water leading to the solvent accessible cavity of LPO; and 3) Characterize the interaction of mucin proteins with lactoperoxidase molecules. Cystic fibrosis (CF) is a particularly menacing disease not only because a large portion of its affected population are children, but also because it is a multi-system disease, making it very difficult to study. Within the airways of those afflicted with CF, lactoperoxidase, an enzyme responsible for attacking invading bacteria, has been shown to be faulty. The goals of this work are to gain a more accurate understanding of how this enzyme works and ultimately provide future directions for more targeted therapies. [unreadable] [unreadable] [unreadable]