Although Ras plays a fundamental role in cellular proliferation, differentiation and transformation, clear functional differences between the three major Ras proteins (N-, H- and K-Ras) have not as yet been demonstrated. In this study, chimeric constructs were used to compare directly transformation by N-, H- and K-ras oncogenes. In Rat-2 and NIH3T3 fibroblasts, transformation assays (anchorage independence, focus-formation and growth in 1% FCS) showed that H12-Ras was more transforming than N12-Ras or K12-Ras. By contrast, in the human multipotent haemopoietic cell line, TF-1, N12-Ras exhibited greater biological activity. Northern blotting and protein analyses indicated that these findings were not the result of differences in expression or stability of p21Ras. Using further H-ras/N-ras chimeric constructs, we found that the greater transforming activity of H12-Ras in fibroblasts was not due to the hypervariable-CAAX region, but rather to unique sequences between amino acids 84 and 143. These data demonstrate cell specific differences in the intrinsic transforming potential of N-ras, H-ras and K-ras oncogenes.