Immortal cell lines arose spontaneously during in vitro culture of initially normal fibroblasts, MDAH041 and MDAH087, from patients with Li-Fraumeni familial cancer syndrome. Fibroblasts from a control donor, MDAH170, maintained a normal morphology and senesced at 31 population doublings. The immortal fibroblasts have several properties of transformed cells. In addition to having acquired an altered morphology and chromosomal anomalies, MDAH041 and MDAH087 have escaped from senescence, growing beyond 300 and 100 population doublings (pd), respectively. As early as 50 pd, these cells can be transformed by an activated H-ras oncogene to form tumors in nude mice. However, MDAH041 immortal cells were resistant to tumorigenic transformation by transfection with the v-abl oncogene.