Isoprinosine (NPT-10381) was given orally to a group of 22 volunteers at a daily dose of 6 g for 7 days; a control group of 23 volunteers received placebo. Volunteers were inoculated intranasally with both rhinovirus type 9 and rhinovirus type 31, and the clinical picture, extent of virus shedding, and serological responses were assessed. There was no evidence that the compound had useful antiviral activity under the conditions of this trial.