The fusion of Sendai virus with phosphatidylserine vesicles was monitored by a pyrene-phosphatidylcholine fluorescence assay. A strong influence of pH and ionic strength on the extent of fusion was observed. The negatively-charged polymers (dextran sulfate, heparin and chondroitin sulfate) inhibited the ability of the viruses to fuse with the liposomes. The extent of inhibition, for a given amount (w/v) of the polymers, was the greatest for dextran sulfate followed by heparin and chondroitin sulfate. The extent of inhibition depended on the pH and ionic strength of the solution; the lower the pH of the solution, the more effective the fusion inhibition by the polymers. The molecular weight of dextran sulfate (DS) influenced the inhibition effect, i.e., DS with higher molecular weight exhibited a stronger inhibition effect. The presence of sodium sulfate, even in excess concentration, had no inhibitory effect on fusion. On the other hand, PEG had an opposite effect on fusion compared to the negatively-charged polymers, and it decreased their inhibition effect when both were present in the same media. It is concluded that the inhibition of the fusion activity of Sendai virus results from the adherence of negatively-charged polymers to the virus surface preventing close contacts between the virus and liposome surface.