Hepatitis A virus (HAV) strain HM175 derived after repeated subculture of persistently infected B-SC-I cells caused a specific cytopathic effect upon acute infection of B-SC-1 cells. The virus formed visible plaques on B-SC-I cell monolayers after 9 to 14 days of incubation at 34°C. and virus can therefore be titrated by plaque assay. Virus could be re-isolated from plaques of infected cells. which allows the clonal isolation of HAV variants. The stability of a plaquepurified variant of HAV at elevated temperatures exceeded that of poliovirus type 1. but this variant is less stable than previously reported strains of HAV.