The pathogenicity of 3 local isola of paeudorabies virus, named P7, TNL and C2A, were analysed aftr passaging 14 times in LTK-cells with BUdR Results indicati that P7 and C2A were still highly virulent to day-old chicks. On the other hand, the pathogenicity of TNL was greatly reducal and kecame avirulent after? more passages with ara-T. This TK-negative mutant could not rephca in the medium containing HAT. Furthermore, the TK-negative mutant could not Ire ravered from train, trigeminal nerve and some other organs of pigs inoculaal either intramuscularly IIllhEkfl7 or intanasally. The pathogenenathy of the 21st pessaged TK-mutant of TNL stain to wedr-old suckling piglets was also stidiaL Result showed that it was avirulens, no laency was found. All vaccinated pigs survived and all sentinel and control pigs were killed when they were challenged with a virulent strain 43 days afr vaccination. Average periods of fever, virus shedding aft challenge and neutrabng antikidy response kefoxe challenge indicaal that intramuscular vaccination could induce ketter gotection than intanasal vaccination.