The s.c. infection of DBF-1 mice with HSV-2 has a tumor enhancing effect on simultaneously i.m. implanted MCA-induced syngeneic spindle cell sarcoma. Thus with the dosage used here, the first palpable tumors appeared on day 6 in the virus treated group as compared to day 9 p.i. in the sham treated batch. Further, more tumors were formed: 75% among the infected and 40% among the sham injected. The tumor yield could be modulated: no difference between the 2 groups was found when the tumor cell dose was increased sufficiently to advance the first appearance of tumors to less than 6 days. The same result was obtained when the neoplastic cells were implanted 3 days ahead of injection of the virus. When the dose of the cells was decreased, no tumors were found in the sham treated batch, whereas there were some in the infected group. Previous in vitro mixing of the neoplastic cells with virus completely prevented tumor formation, probably due to their lysis by the virus. In fact, in tissue cultures, MCA cells become lytically infected with HSV-2. It is speculated that a similar situation occurs in acutely ill mice which die before the 13th day p.i., i.e., the tumor cells are destroyed by the virus. In fact, those animals, as a rule are free of neoplasmas. Thus it appears that an acute HSV-2 infection prevents MCA sarcoma whereas a latent infection promotes its growth. The possible reasons for this tumor enhancement caused by HSV-2 infection are discussed.