Denaturing gradient gel electrophoresis has been applied widely for the detection of mutation(s) and polymorphisms of various genes. It is shown that this system is also feasible for analyzing mutations in the thymidine kinase (tk) gene of herpes simplex virus type 1 (HSV-1). Thus, this system is applicable for examining whether a drug-resistant HSV-1 contains a tk mutation(s) or otherwise. This system can also be useful for detecting heterogeneity and the emergence of drug-resistant mutants in clinical samples.