A number of aromatic mono- and bis-amidines are capable of blocking cell fusion induced by Respiratory Syncytial (RS) virus. Suitable amidino compounds include those selected from the group consisting of 1-4-di(4-amidinophenoxy)-2-butanol; bis(5-amidino-2-benzimidazolyl)methane; 1,2-bis(5-amidino-2-benzimidazolyl)ethane; 5-amidino-indole; 5-amidinobenzimidazole, 5-amidino-1-methylindole and 5-amidino-1-(4-amidinobenzyl)indole. The most powerful of the compounds, bis(5-amidino-2-benzimidazolyl)methane (BABIM), is able to achieve complete suppression of syncytium formation at a concentration of 1 .mu.M. Inhibition occurs in RS virus-infected Hep-2 cells as well as CV-1 cells. BABIM also causes a significant retardation of RS virus penetration, but does not interfere with adsorption. Addition of the amidines after the penetration of RS virus does not affect single cycle yields. The compounds may be used in the prophylactic control of RS virus in man.