A cistron specific enzyme-forming capacity method was used to study the control of herpes simplex virus (HSV) specific deoxypyrimidine kinase (dPyK) mRNA synthesis. In this assay, the alpha (or immediately early) protein was required to effect the transcription of dPyK mRNA. However, the dPyK mRNA synthesis was sensitive to alpha-amanitin in alpha-amanitin sensitive cells and resistant to alpha-amanitin in alpha-amanitin resistant cells. The effective dose range of alpha-amanitin used and the genetic lesion in alpha-amanitin resistant cells suggested that cellular DNA-dependent RNA polymerase II was also involved in the transcription of dPyK. This study suggests that two components, the HSV alpha protein and the cellular RNA polymerase, II, were required for dPyK mRNA synthesis.