Ribavirin administered orally at 75 mg/kg/day in 3 divided doses for 10 days, beginning either 1 or 4 days, after aerosol exposure of cats to calicivirus strain 255, failed to have any beneficial effect on the clinical course of the disease or to reduce viral excretion. Indeed, there was enhanced severity of the clinicopathologic findings in the treated exposed group, due largely to hemorrhage resulting from profound thrombocytopenia. Other toxic effects included depression of red and white blood cells, increased alanine aminotransferase activity, icterus, and body weight loss. Toxic effects were largely reversed within 1 week of cessation of treatment.