More than 50 reference strains and field isolates of equine herpesvirus type 1 (EHV-1) were examined by a touchdown PCR. Primers for specific amplification of EHV-1 DNA were chosen from the terminal and internal repeat regions of the EHV-1 genome where the high-passaged live vaccine strain RacH displays symmetric 850 bp deletions. The positive strand and one negative strand primer were designed to encompass the deletions present in RacH, and the second negative strand primer was designed to hybridize within these deletions. Discrimination between field isolates and the vaccine strain was achieved by the generation of amplification products of different size: In all EHV-1 reference strains and field isolates, a 495 bp DNA fragment was amplified specifically, whereas a 310 bp fragment was amplified when DNA of the vaccine strain RacH was used as a template. PCR amplification was only obtained in the presence of 8-10% dimethylsulfoxide and when the primer annealing temperatures were decreased stepwise from 72 degrees C to 60 degrees C. Under these conditions as little as 100 fg template DNA, corresponding to about 100 genome equivalents, could be detected. The PCR assay allows fast and sensitive discrimination of the modified live vaccine strain RacH from field strains of EHV-1 since it is applicable to viral DNA extracted from organ samples and paraffin-embedded tissues. It may thus be helpful for examining the potential involvement of the RacH live vaccine strain in abortions of vaccinated mares.