Transformation of human cells from a thymidine kinase (ATP:thymidine 5'-phosphotransferase, EC 22.214.171.124)-negative to a thymidine kinase-positive phenotype has been achieved by using purified DNA from herpes simplex virus type 2. The specific activity of the DNA was in the range 0.5 to 2.0 transformants per microng and the efficiency of gene transfer was up to 1 transformant per 10(5) recipient cells. Several transformed lines able to grow continuously in medium selective for thymidine kinase-positive cells have been established. All of these lines express a thymidine kinase activity of viral origin but they differ from each other in the stability of enzyme expression. Subclones derived from a given transformed line inherited the degree of stability of the parental line.