To evaluate the effect of an interferon inducer on respiratory tract virus infection, poly l.poly C was administered intranasally to volunteers for a seven-day period beginning one day prior to inoculation of rhinovirus 13 or type A2 influenza virus/Hong Kong/68. In three separate trials, toxic effects were not detected and there was a small, but definite, reduction in symptoms of upper-respiratory tract illness associated with drug treatment. However, there was a variable effect on the pattern of virus infection. In only one of the two rhinovirus studies was a reduction of virus shedding observed. Treatment did not decrease the shedding of type A2 influenza virus. The minimal amounts of nasal interferon stimulated by the intranasally administered poly l.poly C may have been responsible for the less-than-optimal results obtained.