This report describes a new method for detecting and measuring the antiviral activity of volatile compounds. This method, the vapour phase test, is a modification of the conventional plaque reduction test, in that the compounds instead of being incorporated in the overlay medium were deposited on the inner surface of the Petri dish lid. During the incubation period, the compounds (if volatile) permeated the overlay medium before exerting their antiviral effect. Compounds which have or may come to clinical trial against rhinovirus infections were compared by normal plaque reduction assays and by means of this new technique. Flavans were shown able to exert their antiviral activity in the vapour phase and thus display an advantage over non-volatile compounds. The report also briefly shows how the technique was used to evaluate the structure-activity relationships in a series of analogous compounds.