A human colonic adenocarcinoma transforming gene, recently identified as a cellular homolog of the Kirsten sarcoma gene (v-ras), was used to assign the human cellular Kirsten ras2 gene to chromosome 12 by the Southern hybridization method. A single 640 base-pair Eco RI--Hind III fragment of the transforming gene, isolated by DNA transfection and molecular cloning, can detect a single Eco RI fragment (2.9 kilobase pairs) of DNA from phenotypically normal cells. The data suggest a constant chromosomal location of c-Ki-ras2.