The article is devoted to the current problem of differential diagnosis of diseases of viral etiology in farm animals. Viral diseases are currently widespread, occupy a leading role in the infectious pathology of farm animals, causing enormous economic damage. Given the magnitude of animal vaccine prophylaxis, in order to increase the effectiveness of antiepizootic measures, the urgent issue i s the development of methods for the rapid and effective detection and differentiation of field and vaccine strains of the infectious rhinotracheitis virus in cattle. The possibility of using a polymerase chain reaction to identify and differentiate a vacc ine strain from epizootic strains and isolators of the cattle infectious rhinotracheitis virus is considered. In the process of research, a PCR-RFLP analysis method was developed to detect the IRT virus in the test material. The PCRRFLP analysis method was used to identify and differentiate the vaccine strain TK-A form epizootic strains and isolators of the cattle IRT virus. The principle of PCR, based on repeated repetition of DNA cycles, annealing and synthesis, which leads to an increase in the number of specific DNA fragments of the pathogen, allows you to take into account the results of PCR in an agarose gel. Analysis time is about 30 hours. The sensitivity of detecting viral DNA is 1-10 picograms (102 TCD). Due to characteristics such as relative simplicity and reaction rate, high sensitivity, specificity and reproducibility, PCR has recently become widespread in basic and applied research in various fields of biological science, including veterinary virology. The results obtained during the studies show that the use of PCR-RFLP allows to differentiate field and vaccine strains and isolates of the IRT virus with a high degree of reliability. The use of PCR -RFLP analysis increases the efficiency and informativeness of studies in the molecular epizootol ogy of cattle RTI, as it allows not only to identify the DNA of different virus strains regardless of their nature, but also to differentiate between them, including differentiating the strain TK-A used for the production of attenuated vaccines against epizootic strains and isolates of the virus.