SummaryHuman serum specimens containing respiratory syncytial virus (RSV)‐specific neutralizing antibody were found to prevent the formation of syncytia when applied to HEp‐2 tissue culture monolayers which had been infected with RSV 12 h previously. This was evidenced by the demonstration of RSV‐infected cells without any syncytia formation in the monolayers treated with RSV antibody‐positive serum. On the other hand, widespread syncytia formation was observed with antibody‐negative control serum. The inhibitory effects of RSV antibody progressively declined when applied beyond 12 h after infection. Protection of the monolayer against syncytia formation occurred only in the presence of antibody and was quickly lost after the serum was removed. The titer of antisyncytial antibody correlated with the titer of neurtralization antibody.AbbreviationsRSV, respiratory syncytial virusCPE, cytopathologic effectsPBS, phosphate‐buffered salineMEM, minimal essential mediumTCID50, 50% tissue culture infectious dose