In a single line of human foreskin fibroblasts, minimum inhibitory concentrations (MICs) and the minimum intracellular virus inactivation concentrations (MIICs) of arabinosyladenine, arabinosylhypoxanthine, and arabinosyladenine 5'-monophosphate were assayed for a number of recent isolates of herpes simplex virus types 1 and 2 (HSV-1, HSV-2), varicella-zoster virus (VZV), and cytomegalovirus (CMV). (The term MIIC is used here to describe the selective qualitative intracellular inhibition of the virus inoculum in the primary tissue cultures. The inoculum is not recoverable in subcultures free of antiviral agent.) MICs and MIICs of each of the antiviral agents were readily obtained for each strain of HSV-1, HSV-2, and VZV tested, but all seven strains of CMV tested were much more resistant. At the endpoint, there was little variation in the MICs or MIICs among strans of the same virus. Final MIC results for HSV-1 and HSV-2 were complete after 3 days of incubation; CMV and VZV results required as long as 6 days. The MIC for each herpesvirus increased with incubation, but at the endpoint the MIC and MIIC were approximately equal. VZV was most susceptible to each drug, followed by HSV-1 and HSV-2. The latter two viruses were quite similar. There was no difference in antiviral susceptibilities among any of the strains of HSV-1, HSV-2, VZV, or CMV tested. The toxicities of arabinosyladenine, arabinosylhypoxanthine, and arabinosyladenine 5'-monophosphate were simultaneously compared by using both microscopic cytotoxicity and inhibition of uptakes of [14C]thymidine into cellular deoxyribonucleic acid and of 14C-labeled amino acids into protein. The selective inhibition of each antiviral agent against viral and cellular deoxyribonucleic acid polymerases was confirmed. Simultaneous assays of antiviral and anticellular activities of antiviral agents may be useful in projecting further in vivo experiments.