1-(2-Fluoro-4-thio-β-D-arabinofuranosyl)-5-methyluracil exhibits an anti-EB virus activity, and is therefore effective as a prophylactic or therapeutic agent for a disease associated with an EB virus. Each of a plasmid capable of expressing EB virus-TK and a plasmid capable of expressing human-TK is introduced into a TK-defect cell, thereby producing two types of cells respectively having the plasmids introduced therein. By using the two types of cells, it is possible to screen a medicinal agent which has cytotoxicity against the cell having the plasmid capable of expressing EB virus-TK introduced therein but has no toxicity against the cell having the plasmid capable of expressing human-TK introduced therein. In this manner, it becomes possible to screen a medicinal agent which specifically exhibits an anti-EB virus activity.
