Our demonstration that not all infectious viruses are cytopathic in M. dunni and FerrD2 cells supports the idea that virus-cell fusion and cell-cell fusion are distinct receptormediated phenomena. A similar lack of correlation between infectivity and syncytium formation has been reported, for example, in a mouse cell line that is unusual in its resistance to HTLV Env-mediated syncytium formation although it is highly susceptible to virus infection [22] . It has also been shown that, for a transformed NIH 3T3 cell line subject to MoMLV-induced syncytium formation, chloroquine treatment blocks MoMLV entry but does not also block syncytium formation [23] . Our results further distinguish cell fusion and virus entry as separate receptor functions.