Among the many targets for antiviral therapy that arise during certain viral infections are the virus-coded proteinases. These enzymes are required to process virus-specific precursors involved in the maturation, assembly and replication of such pathogenic human viruses as poliovirus, encephalitis virus, hepatitus B virus, and human immunodeficiency virus. These virus-coded proteinases are highly specific for their virus-coded substrates. Thus, if equally specific inhibitors can be developed and targeted to infected cells, they should interfere with virus replication and not with normal cellular metabolism. Human adenovirus 2 encodes a proteinase that processes six virion polypeptides during virus morphogenesis. In the absence of an active virus-coded proteinase, noninfectious virus is produced. Because much is known about the molecular biology of adenovirus 2, and because its virus-coded proteinase is a serine proteinase, adenovirus 2 is a good model system to test the efficacy of proteinase inhibitors as antiviral agents. Of the four major classes of proteinases, serine proteinases and their inhibitors are by far the best characterized. The Ad2 proteinase will be cloned, expressed, purified and characterize. Mutants of bovine pancreatic trypsin inhibitor will be rationally and randomly designed, cloned, expressed and selected as specific inhibitors of the Ad2 proteinase. Bovine pancreatic trypsin inhibitor is chosen, because it has only 58 amino acids, has a high affinity for trypsin-like serine proteinases, and has been cloned and expressed in an active form in E. coli. After selecting for additional properties, mutant bovine pancreatic trypsin inhibitors will be tested for antiviral activity in the adenovirus 2 model system. If successful, the lessons learned from this model system can easily be extended to more medically relevant viruses.