were used as received after dilution in assay buffer or
purified by using a microfiltration device (5,000 rpm, Centricon
30, Millipore). Similar results (sensitivity and selectivity) were
obtained with purified and unpurified samples. Viral concentrations
were measured by transmission electron microscopy
after staining samples with uranyl acetate and by fluorescence
microscopy using 1,1-dioctadecyl-3,3,3,3-tetramethylindocarbocyanine-
labeled viruses.