A method is provided for detecting whether a test composition can inhibit cell fusion mediated by paramyxovirus infection. A method is also provided for identifying an antiviral peptide capable of inhibiting paramyxovirus-induced cell fusion. The methods are based on the discovery that the hemgglutination and fusion-inducing proteins of paramyxoviruses interact to form a complex termed HN:F, that is prerequisite to cell fusion. Antibodies to HN:F provide a novel means of providing improved immunity against paramyxovirus infection.