Viruses were purified from BHK-21 infected cells, as previously described using a discontinuous sucrose gradient [20] . Virus particles formed a blue opalescent band at the interface of the sucrose solutions. This was recovered and further purified by layering onto a continuous PercollH gradient as previously described [21] , using an SW41 rotor (100000 g, 1 hour, 4uC). The virus formed a blue band which was collected, diluted in 0.1 M Tris-HCl and pelleted at 10000 g for 1 hour.