Viruses were isolated from nasal washings of volunteers receiving experimental therapy for rhinovirus type 9 infection with intranasal sprays of a new synthetic antiviral R61837. On a screening test nine subjects yielded drug sensitive virus and four resistant virus. In four others the virus was sensitive at first but became resistant later, while in one the reverse occurred. Evidence is given that at least some of the resistant viruses were present in the respiratory tract and were not selected during virus isolation. Of six viruses studied in detail, five had a low degree of resistance and one was highly resistant. The degree of resistance of the five was similar for an antiviral chalcone, dichloroflavan and disoxaril. The sixth was different in that the resistance to disoxaril was relatively less than to the other drugs. The significance of these results is discussed--these are the first experiments in man to show the selection of drug resistant rhinovirus.