We showed previously that diploid human fibroblasts that express a transfected HRAS oncogene from the human bladder carcinoma cell line T24 exhibit several characteristics of transformed cells but do not acquire an infinite life-span and are not tumorigenic. To extend these studies of the T24 HRAS in human cells, we have utilized an infinite life-span, but otherwise phenotypically normal, human fibroblast cell strain, MSU-1.1, developed in this laboratory after transfection of diploid fibroblasts with a viral v-myc oncogene. Transfection of MSU-1.1 cells with the T24 HRAS flanked by two transcriptional enhancer elements (pHO6T1) yielded foci of morphologically transformed cells. No such transformation occurred if the plasmid containing T24 HRAS had only one enhancer or none at all or if the normal human HRAS gene was transfected in the pHO6 vector (pHO6N1). Cell strains derived from such foci expressed high levels of T24 HRAS product p21, formed colonies in soft agar at high frequency, proliferated rapidly in serum-free medium that does not support growth of the parental cell line, and formed progressively growing, invasive fibrosarcomas. These foci-derived T24 HRAS-transformed cell strains, as well as cells from the tumors derived from them, had the same near-diploid karyotype as that of the parental MSU-1.1 cells. Transfection of pHO6T1 into two other infinite life-span human fibroblast cell lines, cells that had not been transfected with v-myc, also resulted in malignant transformation, suggesting that the infinite life-span phenotype of MSU-1.1 cells, and not necessarily expression of the v-myc oncogene, was the factor that complemented T24 HRAS expression to cause malignant transformation.