As a step toward the achievement of targeted expression of toxic genes, we have established a model system using the selective trans-activation of the late promoter P38 of Minute Virus of Mice (MVMp) by the parvoviral nonstructural protein NS-1. The conditionally toxic herpes simplex virus type 1 thymidine kinase (tk) gene (HSV1-tk) was cloned under the control of the P38 promoter and transfected into NIH-3T3 TK- cells. Treatment of the stably transfected cells with acyclovir (ACV) followed by infection with MVMp reduced cell survival by 3.5- to 5-fold compared to the toxic effects of ACV or MVMp alone. These results indicate that it should be possible to combine the genuine cytopathic action of parvoviruses with a specific activation of toxic genes driven by parvoviral promoters, to achieve the targeted destruction of parvovirus-expressing (in particular tumor) cells.