Infection with herpes simplex virus (HSV) enhances the growth of Lewis lung tumor (LLT). Infection with HSV-1 enhances as efficiently as HSV-2. A significant acceleration of primary tumor formation was obtained. In mice, lung metastases were increased in number and size above the controls. Enhancement occurred only with high doses of infectious virus and was also obtained only when the virus infection was at a site close to that of the tumor implantation. This finding, in addition to the lack of antigenicity of LLT, is interpreted as arguing against an immunologic mechanism of the phenomenon.