Coronaviruses are major cause of disease in several animal species and with the recent emergence of the SARS virus have become increasingly important as human pathogens. We have recently developed the avian virus infectious bronchitis virus (IBV) as a system to study coronavirus entry and have developed fluorescence dequenching assays of viral fusion to show the role of low pH for entry of IBV. We are currently applying these techniques to other members of the Coronaviridae. With IBV as a model, we are exploring the process of fusion activation of the SARS-coronavirus, as well as feline infectious peritonitis virus (FIPV), in order to gain molecular insight into the pathogenesis of these deadly viruses