Superinfection of Raji cells with Epstein--Barr virus (EBV) leads to syncytium formation. Studies using metabolic inhibitors and amino acid analogues suggest that the fusion-inducing factor belongs to the early group of virus-specified proteins. Induction of early EBV protein synthesis in Raji cells by using various chemicals also leads to syncytium formation, indicating that the fusion process is not caused by a virion membrane protein introduced into the cells upon infection. The relevance of these findings to the association of EBV with carcinoma of the nasopharynx is discussed.