VeroE6 cells, mouse 3T3 cells, mouse macrophage-derived J774.1 cells and MRC-5 cells were infected with viruses at an moi of 0.02 at 37uC for 1 h, and washed 3 times with PBS. Then medium was added. Culture supernatants were harvested at various times p.i., and the virus titer was measured by plaque assay.