Intraperitoneal administration of bis(5-amidino-2-benzimidazolyl)methane at well-tolerated daily doses of 25 mg/kg subsequent to challenge and for 3 days thereafter effected over a 1-log reduction in the amount of virus recovered from lungs of cotton rats inoculated intratracheally with respiratory syncytial virus. When animals were immunosuppressed to prolong virus shedding, the reduction in recovered virus achieved with a 7-day dosing schedule of bis(5-amidino-2-benzimidazolyl)methane exceeded 2 logs.