Many chemical substances have been tested for antiviral activity and the author presumably selected the benzo(b)thiopene derivatives because it was known they possessed certain inhibitory biological activities and because they were available. Methods for screening possible antiviral agents tend to be laborious and not very accurate but in selecting the indirect immunofluorescent technique the author could carry out rapid screening of substances in a laboratory where expertise in immunofluorescence was already at hand. Inhibitory activity against three representative viruses was found with many of the 179 substances, in concentrations considered non-toxic for the tissues in which they were tested. Nine of the more promising compounds were selected and an attempt made to find out the specific points of activity leading to inhibition of viral synthesis. For studies on influenza and vaccinia viruses comparisons were made with such well-known substances as amantadine hydrochloride and 5-iodo-2-deoxyuridine. In general it was found that the compounds did not interfere with absorption or phagocytosis of virus but at a later stage which appeared to vary according to the derivative tested. It is concluded that the benzo( b) thiopene nucleus acts as an acceptable carrier and the antiviral activity largely depends on the nature of the side-chain present.