ras oncogene mutations appear in over 50% of colon tumors in humans. Studies in animal systems have revealed that ras mutations are also present in preneoplastic lesions, suggesting the possibility of early detection of ras mutation in morphologically normal colon tissues for diagnostic purposes. An Enriched PCR, developed by us, eliminates most of the normal ras alleles prior to amplification; subsequent analysis via RFLP enables the detection of one mutant allele within 10(4) normal alleles. Using the Enriched PCR, we have determined the frequency of mutant ras alleles in normal mucosae and in adenomatous polyps of patients with or without adenocarcinoma. Of the 42 patients who had colon tumors, 15 were found to harbor K-ras oncogene mutation (35%). In two of the 14 cases with mutant K-ras in the tumor tissue we were able to identify mutations in tissues that had been obtained from a site at considerable distance from the tumor (13%); Analysis of 7 adenomas identified one as a carrier of the mutant ras allele (14%). Of 11 normal colonic mucosa obtained from patients without neoplasia, one specimen contained K-ras mutation. Thus, mutated alleles of K-ras may be present, at low frequency, throughout the 'normal appearing' tissue. Cells of normal appearance that harbor such mutation, have the potential to undergo further changes and to develop into the transformed phenotype. Overall, our findings suggest that mutant ras alleles can be detected in preneoplastic mucosa that is morphologically normal, and in adenomas, suggesting the occurrence of an initiation event, and possibly enabling the identification of patients who may be at high risk for developing malignant tumors.