4\',6-Dichloroflavan, a potent inhibitor of rhinovirus replication in tissue culture systems was tested in a double-blind, placebo-controlled volunteer trial for its protective efficacy against experimental rhinovirus infection. Dichloroflavan was administered intranasally as a 5 per cent w/v aqueous suspension (40 mg; 5 times per day) for 5 doses before and 21 doses after intranasal challenge with rhinovirus type 9, a virus type known to be highly sensitive to the drug when tested in tissue culture. A total of 49 volunteers were included in the efficacy analysis. Dichloroflavan did not produce any consistent or significant reduction in clinical or laboratory parameters of infection. Indeed there was some indication that treatment with the drug may have been associated with increased severity of clinical signs and symptoms. Dichloroflavan administered intranasally is not, therefore, of value in the prevention of human rhinovirus infection.