Abstract: To develop a rapid, highly-sensitive assay method to detect rare mutation of Ki-ras g ene, and to apply it in the detection of colorectal tumors. Methods By using an assay method named PCR-PIREM A( Primer Introduced Rest riction w ith Enrichment of Mutant Allele Assay), thirty samples of colorectal tumors w ere analyzed. Results Eight ( 26. 7% ) of thirty contained mutations. The sensitivity of this method is confirmed to be 10 - 5 ～ 10 - 6 through the reconst ruction experiments. Conclusion PCR-PIREM A is a rapid, highly-sensitive assay. It is necessary for early detection of cancers with highly f requency of known point mutations and kinetics studies of mutagenesis induced by carcinogens at particular bases of a gene.