Hantaviruses do not produce cytopathic effects (CPE) in cell culture. However, a syncytial CPE can be induced in 7-day cultures of hantavirus growing in Vero E6 cells by reduction of the pH to approximately 6.2 using a HEPES based buffer. The appearance of this acid induced CPE was examined for seven different hantavirus strains. The differences noted were striking and reflected the taxonomic differences between hantaviruses. At 10-100 TCID50% the size of syncytial foci was very large for Seoul type viruses and smallest for Puumala viruses. The size of syncytia for Hantaan (HTN) virus was intermediate between Puumala (PUU) and Seoul (SEO) type viruses.