The frequency of mutants among the descendants of intact parvovirus H-1 was increased when cells of human and rat origin were exposed to UV light prior to infection. An enhanced mutagenesis of intact H-1 could be induced to almost the same extent in unirradiated cells by preinfecting them with H-1 or simian virus 40 that had been irradiated with UV light. The expression of an enhanced mutagenesis of unirradiated H-1 after cell treatment with UV light or UV-irradiated virus was delayed, transient, and dose-dependent, and it was dependent on de novo protein synthesis.