The mutagenic activity of native and inactivated herpes simplex virus (HSV-1) has been studied. The native HSV-1 strain has been shown to induce the gene mutations of 6-mercaptopurine resistance in Chinese hamster cells. The 1.5- to 2-fold decrease of mutagenic activity is due to HSV inactivation by thiophosphamide and formalin treatment. These data indicate the possibility of the given agents application for decreasing of the genetic danger of antiviral vaccine.