Induction of acyclovir resistance was studied in the immune compromised host by multiple passage of plaque-purified herpes simplex type I strains in athymic nude mice receiving suboptimal antiviral therapy. Mice infected with a highly pathogenic clinical isolate rapidly developed infections that were resistant to therapy. Viruses isolated from these mice had decreased in-vitro sensitivities to acyclovir, as well as altered characteristics when assayed by [125I]plaque autoradiography. In contrast, less virulent laboratory strains, or a genetically stable clinical isolate, showed no indication of mutation to resistance after extended passage in this mouse model. Highly pathogenic viruses may increase the probability of mutation to resistance because of the large amount of infectious virus they produce, while viruses of equivalent virulence may produce different amounts of drug-resistant progeny because of alterations in the replication fidelity of the viral DNA polymerase.