Haemorrhagic fever with renal syndrome viruses, are members of the family Bunyaviridae. They cause cell to cell fusion from within under acidic conditions. This phenomenon was found to occur under a pH range of between 4.9 to 6.3 for all the viruses examined. The pH range which causes cell fusion was similar to that reported for the La Crosse virus of the Bunyaviridae, hence indicating that this property is a common biological characteristic among this family of viruses. Titration of virus infectivity and neutralizing antibody was done by counting the number of fused cell foci produced in infected Vero cell monolayers after low pH treatment. This method was simpler and more rapid than the ordinary plaque formation method or that of counting infected cell foci by IFA or immunoenzyme assay. In addition, this method may also be applicable in the detection of other enveloped viruses which do not cause a typical cytopathic effect.