Cell fusion reaction with HVJ includes both cell membrane-cell membrane fusion and HVJ envelope-cell membrane fusion. The present paper shows that two kinds of fusion were separated by use of medium containing 0.38 M or more glucose, mannose or galactose, or 0.25 M or more sucrose. In these media, the process of viral envelope-cell membrane fusion started on incubation at 37°C, but it stopped at an early stage, so that subsequent reactions, such as cell-to-cell fusion, did not occur. The reason for this is unknown, but may be related with the finding that these saccharides strongly inhibit cell membrane movement. Thus, when cell aggregates produced by adsorption of HVJ were incubated at 37°C in media containing these saccharides, they disaggregated into single cells fused with viral envelopes within 30 min. When these single cells were mixed with untreated cells, the former agglutinated with the latter without further addition of HVJ at 0°C and the cells in the resulting aggregates fused on incubation at 37°C. The cells fused under these conditions are to be all heterokaryons, not homokaryons.