The addition of 5 mM dithiothreitol to a cell-free assay system for influenza ribonucleic acid (RNA) polymerase activity reverses the inhibitory activity otherwise possessed by three established antiviral compounds: selenocystine, 4-(2-propinyloxy)-beta-nitrostyrene, and acetylaranotin. Although 50% or greater enzyme inhibitory activity is repeatedly achieved for these compounds at a concentration of approximately 50 mug/ml (0.1 to 0.25 mM) in the absence of dithiothreitol, no inhibition is seen in its presence at inhibitor concentrations as high as 200 mug/ml. Against the deoxyribonucleic acid-directed RNA polymerases of Escherichia coli and chicken embryo cells, acetylaranotin and 4-(2-propinyloxy)-beta-nitrostyrene caused very little inhibition. Only selenocystine significantly inhibited these two enzymes in the absence of reducing agent, but to an extent substantially less than that obtained against the viral enzyme. These results appear to suggest that influenza RNA polymerase is uniquely sensitive to a variety of structurally diverse antiviral compounds as a consequence of their sulfhydryl reactivity-a fact which might aid in the search for and development of more potent chemotherapeutic agents.