Abstract: 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.