Melanins are implicated in a variety of important biological processes yet they remain enigmatic polymers. For several oopportunistic pathogenic fungi including Cryptococcus neoformans, melanin synthesis is associated with virulence. C. neoformans is a major cause of meningoencephalitis in patients with AIDS. Melanin synthesis has been demonstrated to occur in C. neoformans during infection. The mechanism by which melanin contributes to virulene is poorly understood but may involve interference with host effector mechanisms. Apart from their importance in microbial pathogenesis, there is evidence that melanins reduce the efficacy of certain antifungal drugs implying that this pigment can be targeted for antimicrobial drug development. In fact, there is evidence that compounds which interfere with melanin polymerization can be therapeutic in murine experimental infection. Melanin is immunogenic and can elicit antibodies that protect against experimental C. neoformans infection. Antibodies to melanin cause cell growth arrest after binding to melanized cells indicating a novel mechanism of antibody action against fungi. The goals of this application are to better define the mechanisms by which melanin contributes to virulence, to investigate the role of antibody responses to melanin and laccase in host protection and correlate biological activity with structure by probing the structure of melanin using labeled substrates. Three aims are proposed: 1) to establish the mechanism by which melanin promotes the virulence of C. neoformans in vivo, 2) to establish the protective efficacy of antibody to C. neoformans melanin and laccase and the mechanism of antibody action, 3) to investigate the structure of C. neoformans melanin using biosynthetically incorporated substrates. It is anticipated that the work proposed here will provide important insights into the biology of melanin and in doing so highlight novel strategies for antimicrobial drug development.