Abstract: Sequential exposure of mouse embryo cells to HSV-2 and TPA gave rise to a synergistic enhancement of the transformation frequency. The transformants were selected for their ability to form dense foci of cells in medium containing 10% or 1% (low) fetal bovine serum. The average number of foci induced with HSV-2 followed by TPA was about 3 or 5 (in low serum) fold greater than that induced with HSV-2 alone. HSV-2 antigen could be detected in about 10% of transformed cells before 27th passage with immunofluorescence technique. Of two cell lines established from single focus, one designated BL which was preferable to form foci in subcultures was tumorigenic after 21th passage. All of the tumors were sarcomas with interlacing bundles of pleomorphic fibroblasts. The other, designated NP was nontumorigenic until 50th passage. The BL cell line was composed of two distict cell types, i. e., pigmented and unpigmented. No viral DNA sequences were detected in the cells of tumors derived from BL cell line.