Propagation. An RF RV strain, as previously described in [27, 28] , was used in all tests. Confluent cultures (in 25 cm 2 Corning flasks) of H4, CLAB, and CIEB cells were washed twice in single strength phosphate buffer (1 × PBS) to remove the foetal bovine serum (FBS) before infection with RV (previously passaged 5 times on H4, CLAB, and CIEB cells in rotation; at 1 × 10 7.5 /mL TCID 50 in the presence of 1 μg of trypsin (Sigma Chemical Co., St. Louis, Mo) per mL. After a 1-hour adsorption period at 37 • C, DMEM, containing 0.5% FBS (Sigma Chemical Co., St. Louis, Mo.) and 1 μg of trypsin per mL, was added to each flask. The flasks with cells were incubated for 24 to 48 h at 37 • C, under 10% CO 2 until cytopathic effect (CPE) was observed by microscopy. When the cells showed extensive cytopathic degeneration, the flasks with cells and virus were subjected to two freeze-thaw cycles to detach and break the cells. The supernatant fluids containing detached cells and extracellular virus were removed, and then the cellular debris was removed by centrifugation at 3500 g for 10 min. Virus was stored at −70 • C until time for tests.