Phosphonoformate was found to be an inhibitor of the deoxyribonucleic acid polymerase induced by the herpesvirus of turkeys. The apparent inhibition constants were 1 to 3 muM. Phosphonoformate was also able to block the replication in cell culture of Marek's disease herpesvirus, the herpesvirus of turkeys, and herpes simplex virus. It was as effective as phosphonoacetate. Phosphonoformate was not an effective inhibitor of a phosphonoacetate-resistant mutant of the herpesvirus of turkeys nor of its induced deoxyribonucleic acid polymerase.