a method for diagnosing hsv infections in human or non - human patients which involves intravenously administering to the patient a non - invasively assayable ligand - labelled anti - viral drug , wherein the drug is a compound which is phosphorylated bby the hsv - induced thymadine kinase present in hsv - infected cells to a phosphorylated assayable ligand - labelled compound which cannot diffuse from within the infected cells and then non - invasively assasying the presence or absence of the accumulated phosphorylated compound present therein . a pharmaceutical composition in intravenously administrable form and adapted for the diagnosis of hsv infection which is a mixture of a non - invasively assayable ligand - labelled anti - viral drug and a pharmaceutically acceptable carrier therefor .