A simple procedure for type differentiation of herpes simplex virus with the use of polymerase chain reaction (PCR)-amplified DNAs, was established: 1. The target sequence region for PCR was chosen from the coding sequences for an envelope protein, with the terminal sequences for PCR primers to be common among different types, but with the internal sequences to be variable. 2. Biotin-labelled probes for each type were prepared by PCR with the above primers and the templates from standard viruses of different types. 3. With templates from isolated strains or clinical specimens, the target DNA segment was amplified, and then immobilized on microplate wells. 4. Hybridization was carried out with the biotin-probes under a stringent condition so that the immobilized DNA was hybridized only with the homologous-type probe. 5. This hybridization result was visualized by using streptavidin-conjugated peroxidase and coloring reagents. This procedure may be applicable to differentiation of types or strains belonging to a group of closely related viruses.