A unique seven-membered heterocyclic-ring inhibitor of adenosine deaminase was studied. One preparation of the compound inhibited replication of herpes simplex virus in the absence of adenine arabinoside. In this capacity, the minimal inhibitory concentration of deaminase inhibitor for herpes simplex virus type I (HSV-1), with 50% reduction of plaque-forming units as the end point, was 37.7 µg/ml. This activity compared favorably with the inhibitory activity of ara-hypoxanthine (34.1 µg/ml). Another preparation of deaminase inhibitor lacked antiviral activity. On the other hand, the adenosine deaminase inhibitor was active at a concentration of 0.009 ILg/ml as a potentiator of the inhibition of HSV-1 by adenine arabinoside. The potentiation of adenine arabinoside by deaminase inhibitor is about 4,000 times more potent than the activity of the direct inhibitory effect on HSV-1. The nature of the possible contaminant of the preparation in question is unknown. Coformycin, another inhibitor of adenosine deaminase, had no antiviral activity in the absence of adenine arabinoside.