Varicella zoster was isolated from the vitreous of a patient with the acute retinal necrosis (ARN) syndrome. We utilized a plaque reduction assay to determine the in vitro susceptibility of the ARN isolate to 6 antiviral drugs. The effective doses for 50% inhibition of plaque numbers were 5.3 uM for acyclovir, 4.7 uM for DHPG, 8.7 uM for Ara-A, 100.7 uM for phosphonoacetic acid, 0.07 uH for BVdU and 2.4 uM for IUdR. Similar inhibitory values were obtained for the OKA vaccine strain of varicella zoster virus. These data do not support the notion that the ARN strain may represent a mutant of varicella zoster virus with significant alterations in either the viral thymidase kinase or DNA polymerase genes based upon its antiviral sensitivities. The implications of these results regarding the role of antiviral chemotherapy in the ARN syndrome are discussed.