Nonmalignant diploid human fibroblast cells (GM3498B) derived from a skin biopsy of a patient with Bloom's syndrome have been transformed by transfection with DNA from a tumorigenic mouse cell line (Ha-8) carrying a single copy of the Harvey murine sarcoma virus (Ha-MuSV) genome. The transformed cell lines have an extended life-span, form colonies in agarose, and proliferate in nude mice--characteristics of neoplastic transformation. Like the parental cells, they also exhibit a high spontaneous level of sister chromatid exchanges. Finally, the transformed cells contain most, if not all, of the Ha-MuSV genome as well as the human rasH sequence. These experiments show that these diploid nonmalignant human cells can be used as recipients in transfection experiments for studying the genetic control of neoplastic transformation.