Virus titers were determined by plaque assay in Vero cell monolayers grown in 24-well plates. For the viral plaque assay, serial tenfold dilutions of each specimen were prepared in HBSS with 2% fetal bovine serum added (HBSS-FBS), and 50 ml of each dilution was adsorbed on duplicate cell monolayers for 1 hour at 37uC. After 1 hour, the monolayers were overlaid with 0.6 ml of a mixture of 1 part 1% agarose and 1 part 2X Eagle's basal medium with Earle's salts, 17 mm HEPES, 8% fetal bovine serum, 100 U of penicillin/ml and 100 mg of streptomycin sulfate/ml. After incubation for 72 h at 37uC in a 5% CO 2 atmosphere, each cell monolayer was stained by adding 0.6 ml of a second overlay, identical to the first, but containing 4% neutral red. After an additional 24 h incubation, plaques were enumerated and viral titers were calculated.