Although an important contribution of ERK and JNK mitogen-activated protein kinase (MAPK) activation in Ras transformation of rodent fibroblasts has been determined, their role in mediating oncogenic Ras transformation of human tumor cells remains to be established. We have utilized the human HT1080 fibrosarcoma and DLD-1 colon carcinoma cell lines, which contain endogenous mutated and oncogenic N- and K-ras alleles, respectively, to address this role. Study of these cells is advantageous over Ras-transformed rodent model cell systems for two key reasons. First, the ras mutations occurred naturally in the progression of the tumors from which the cell lines were derived, rather than due to overexpression of an exogenously introduced gene. Second, although these tumor cells possess defects in multiple genetic loci, it has been established that mutated Ras contributes significantly to the transformed phenotype of these cells. Clonal variant lines of HT1080 and DLD-1 have been isolated which have lost the oncogenic ras allele and exhibit a corresponding impairment in growth transformation in vitro and in vivo. We found that upregulation of Raf/MEK/ERK and JNK correlated with expression of oncogenic Ras in HT1080, but not DLD-1 cells. Furthermore, inhibition of ERK activation in parental HT1080 cells caused the same changes in cell morphology and actin stress fiber organization seen with loss of expression of activated N-Ras(61K). Thus, we suggest that constitutive activation of the Raf/MEK/ERK and JNK pathways is necessary for Ras-induced transformation of HT1080 but not DLD-1 cells. These results emphasize that cell type differences exist in the signaling pathways by which oncogenic Ras causes transformation.