Minimal inhibitory concentrations of 5-iodo-2′-deoxyuridine, 9-β-D-arabiilofuranosyladenine, 9-β-D-arabinofuranosylhypoxanthine,and human leukocyte interferon were determined in an in vitro Vero tissue culture system with the Ket. strain of herpes simplex virus, type 1. These values were 1.1 μg/ml, 22.7 μg/ml, 34.1 μg/ ml, and 91 units/ ml, respectively. The action of combinations of these agents against the Ket. strain was studied. An additive effect was noted when 9-β-D-arabinofuranosyladenine was combined with 9-β-D-arabinofuranosylhypoxanthine, but this effect was not observed when 9-β-D-arabinofuranosyladeninewas combined with 5-iodo-2′-deoxyuridine. Human leukocyte interferon and 9-β-D-arabinofuranosyladenine were synergistic, a fact suggesting that this combination might be given in experimental infections with type 1 herpes simplex virus in animals and that it might eventually be tested in controlled clinical trials in human subjects.