LCMV plaque assay. LCMV titers were determined as previously described (7, 17) . Briefly, peritoneal lavage fluids were thawed from storage at Ϫ80°C and serially diluted. Samples were overlaid onto Vero cell monolayers and incubated at 37°C for 1.5 h. Plaques were allowed to form upon overlay of samples with a 50:50 mixture of agarose-medium 199 (Sigma) for 3 days and then visualized using neutral red staining (Sigma). Included in each assay were positive LCMV control samples and negative controls (uninfected animals).