Some cloned laboratory mutant strains of herpes simplex virus type I resistant to acyclovir H-phosphonate were studied. As was shown for all the clones, the mutations augmented the susceptibility to cidîfovir. Thymidine kinase of the mutant viruses partially retained the capacity to phosphorylate thymidine, but lost the capacity to phosphorylate BVDU. © 2011 Pleiades Publishing, Ltd.