Activated N-ras gene was isolated from human hepatoma tissue by DNA transfection assay coupled with the neomycin selection method and molecular cloning and a point mutation in the codon 61 (CAA----AAA) was noted. However, examination of the proportion of the mutated N-ras gene in the tumor part by molecular cloning and by hybridization using synthetic oligonucleotide probes indicated that the mutated gene occurred with very low frequency. The activated N-ras gene appears located only in a small fraction of the tumor cells. The experimental data indicate activation of this gene as possibly not the major cause of carcinoma, but rather a manifestation of tumor heterogeneity.