Herpes Simplex Virus Type 1 (HSV-1) is a virus commonly existent in nature. Others reported that herpesvirus DNA probably activates type C virus in vitro. L6565 Murine Leukemia Virus (L6565 MuLV) is a kind of virus inducing the murine Leukemia. The main aim of the study was to explore the activation of HSV-1 to the pathogenicity of L6565 MuLV in vivo. Suckling mice were inoculated with either L6565 MuLV & inactivated HSV-1, or L6565 MuLV alone. In order to determine the infection of the mice, HE staining and RT-PCR techniques were applied. The result shows that the L6565 MuLV RNA was detected in 8th week in the mice of MuLV plus HSV-1 group, while delayed to 10th week in the L6565 MuLV alone group. The infection rates of the two groups were 74.3% and 41.7%, respectively. Also, L6565 MuLV RNA was detected in the thymus one week after the inoculation, and it was detected in both thymus and spleen two weeks after inoculation. It is obvious that HSV-1 promoted the infection of L6565 MuLV and increased the incidence rate of murine leukemia induced by L6565 MuLV.