We have recently developed a new method to detect and characterize single base substitutions in transcribed genes which is based on the ability of RNAse A to recognize and cleave single base mismatches in RNA:RNA heteroduplexes. The RNAse A mismatch cleavage assay was applied to screen human colon carcinoma cell lines and primary tumors for the presence of mutant c-K-ras oncogenes. We have determined that the mutant c-K-ras allele is overexpressed and amplified relative to the normal in the SK-CO-1 human colon carcinoma cell line. The oncogene mutation has been characterized by this method as a glycine to valine substitution at codon 12 of the c-K-ras gene. This result was confirmed by cloning and sequencing. We have previously reported that about 40% of primary human colon tumors contain c-K-ras genes mutant at codon 12 (Forrester et al, Nature 327: 298, 1987). We report here the characterization by molecular cloning and sequencing of the mutation in the c-K-ras oncogene from two of these tumors (tumors 3 and 28). We also describe the histopathological characterization of these two tumors and demonstrate, by Southern blot hybridization of NIH3T3 transformants, the simultaneous presence of mutant c-K-ras and N-ras oncogenes in villous adenoma 28. Our results provide evidence for the frequent association of ras somatic mutational activation in the early stages of tumor development in this common type of human cancer.