SUMMARY A plaque assay was developed for titration of infectivity of hemagglutinating encephalomyelitis virus (hev). Viral infection in cell cultures was kept localized by adding nutrient medium containing 1% agar to the cultures. The medium was subsequently removed, and hemadsorption was used to detect otherwise macroscopically inapparent foci of infection. Of several types of erythrocytes tested, those of rats, hamsters, and mice were most suitable for adsorption to infected cells. Conditions required for routine application of the assay were determined.