The survival of UV-irradiated herpes simplex virus on UV-irradiated Vero cells was increased over that on unirradiated cells. A time period between irradiation of the host cells and infection with virus was needed to achieve maximum reactivation. In parallel experiments in which the frequencies of occurrence of the forward mutation in the thymidine kinase gene of the virus were measured, growth of herpes simplex virus on UV-irradiated cells yielded progeny virus that had higher frequencies of TK- mutants than did progeny from infections of control cells. The time course of development of this mutagenic effect was the same as that for the development of the UV-reactivation capacity. Furthermore, development of the UV reactivation could be blocked by inhibition of protein synthesis. These results suggest that an "error prone" inducible UV-reactivation phenomenon exists in mammalian cells.