Topically administered CP-20,961 is known to stimulate nasal interferon. Studies in volunteers given the drug prior to challenge with rhinovirus have yielded both fairly good results and only fair or poor results. This study was undertaken in an attempt to settle the differences between the results of these trials and to evaluate the possible effectiveness of CP-20, 961 when given after virus challenge. Sixty volunteers were randomly divided into four groups. One group received placebo, the second received the drug on the day before and the day of challenge, the third was given the drug for two days beginning 24 hr after challenge, and the fourth received the drug for two days beginning 48 hr after challenge. The average number and severity of symptoms in the group that received CP-20,961 prior to challenge were about half of those in the control group. There was no decrease, however, in the number and severity of symptoms in the groups that received the drug after challenge.