Development of a safe and effective vaccine against human respiratory syncytial virus, and its close bovine relative, remains a high priority. These viruses are the most important cause of severe respiratory infections in very young children and calves. We intend to use recently developed techniques for rescuing negative- stranded RNA viruses from cDNA copies of the entire genome. This approach offers the possibility of using highly developed genetic engineering techniques for introducing desired mutations in the viral genome. Minigenome amplication and alternative assays will be used for rapid screening of promising mutations that may be then transferred to full-length cDNA clones for virus rescue. Mutant viruses will be tested in cell and organ cultures for growth and induction of key regulatory molecules. Viruses with interesting phenotypes will be tested in animal models, including calves as a natural host of bovine respiratory syncytial virus.