This study was applied on a cattle farm of Holstein cows at Sharkia Governorate for the isolation and identification of Bovine herpesvirus type-1 (BoHV-1) and to examine its effect on T lymphocytes. The results of clinical examination revealed that there were respiratory disorders in 30 out of 150 (20%) of cattle including elevated body temperature (40 to 42°C), nasal and ocular discharges, some animals developed severe rhinitis, conjunctivitis, corneal opacity, cough and diarrhea. Out of 30 nasal swabs, 15 swabs were positive for virus isolation as indicated by cytopothic effect (CPE) on MDBK cells. Only 11 of the 15 isolates were confirmed by virus neutralization test (VNT) as BoHV-1 isolates. In addition, only 3 out of 4 BoHV-1 isolates were detected by PCR. Peripheral blood T lymphocytes (PBTL) were analyzed using electron microscopy and comet assay to examine the effect of BoHV-1 on lymphocytes. Electron micrographs of T lymphocytes revealed peripheral condensation of chromatin and fragmentation of the nucleus end of the cell leading to the formation of apoptoic bodies. Comet assay denoted fragmentation of cellular DNA. It could be concluded that BoHV-1 can infect T lymphocytes of cattle, causing directly and indirectly apoptosis which subsequently lead to suppression of cellmediated immunity, enhancing establishment of latency and increasing the probability for secondary bacterial infection.