Six controlled trials of a topical interferon inducer, N\',N\'-diotadecyl-N\',N\'-bis-(2-hydroxyethyl) propanediamine (CP 20961), against rhinovirus infection in volunteers were reviewed. Controls were the drug vehicle (which consisted of a mixture of polysorbate-80 in gylcerol and saline), saline only, and oral lactose. Multiple doses of the inducer given before challenge in any of three regimens reduced the symptomatic response to viral infection. Treatment with three doses of drug the day before challenge enhanced the interferon response to virus; four days of treatment with either the drug or its vehicle before challenge produced a refractoriness to further interferon induction. Both the drug and its vehicle were capable of inducing nasal interferon. When interferon was present at the time of viral inoculation, symptoms were reduced with or without an increase in the interferon response to infection. The rates of infection and illness were increased after a single dose of drug and decreased after a single dose of the vehicle alone.