1-beta-D-Ribofuranosyl-1,2,4-triazole-3-carboxamide (ribavirin) at a dosage of 300 mg/day, or disguised placebo was administered to patients in a closed population during an epidemic outbreak of influenza in Mexico City. Treatment consisted of capsules given three times daily for 5 days beginning with first signs of disease. The study was conducted in a double-blind fashion. Clinical manifestations of the disease, as well as in titer of recoverable virus and in specific serum antibody titer were significantly reduced in the ribavirin-treated group. None of the individuals in the study had any sign of toxic effects attributable to the drug. In the study, 21 patients received the drug and 24 received placebo.