1. Autoradiographic study of syncytium formation between rat XC cells and MuLV-producing cells indicated that the syncytium consisted of XC cells exclusively. Virusproducing cells did not coexist with XC cells in the syncytium. 2. Human cells which formed a syncytium in response to SSAV or BaEV were transformed cells and were not normal cells. SSAV induced over a narrower range of human transformed cells than BaEV. In nonhuman cultured cells, BaEV did not induce a syncytium at all as far as tested. On the other hand, SSAV induced a syncytium in XC, GMK and CV-I cells. Anti-SSAV serum specifically blocked syncytium formation by SSAV but not by BaEV. Similarly anti-BaEV serum specifically blocked syncytium formation by BaEV but not by SSAV. Therefore, syncytium formation is specific for each virus and there is no cross-neutralization between SSAV and BaEV.