Members of the ras family of proto-oncogenes code for 21,000-dalton molecular weight protein products (p21s). Transformation of cells from the normal to the malignant phenotype in experimental studies has been associated with point mutations within the coding region for these ras proteins. Recent reports demonstrate that 40% of human colon cancers and 20% of acute leukemias contain ras mutations in the twelfth or thirteenth codon that can result in amino acid substitutions at these positions in the p21 products. Similarly, studies of ras mRNA detected 40% of human colon tumors with twelfth codon c-Ki-ras mutant mRNA. The authors previously developed a nonradioactive double-antibody enzyme-linked immunoblot assay (ELIBA) for detection of normal and mutant ras p21. They have adapted that technology to specifically detect twelfth codon activated ras p21 utilizing mutation-specific antisera. In this report the authors show that one of seven de novo human bladder cancers and four of seven colon cancers express a twelfth codon activated ras p21. These results document that mutations at both the DNA and mRNA levels are ultimately translated into an abnormal protein product present in human tumors.