Mutation-specific PCR amplification, a special version of allele-specific amplification, is a method for detection of known mutation. In this method, one of the paired primers has a base at the 3' terminal that is complementary to a mutated base. With this primer, only alleles which have the same mutation can be amplified. We analyzed point mutations of N-ras protooncogene codon 61 in thyroid neoplasms using this mutation-specific PCR amplification method. This method is easy to perform, and many samples can be treated at one time. We detected 2 point mutations of N-ras codon 61 out of 7 follicular carcinomas. One was a cytosine to adenine substitution mutation at the first base, and the other was an adenine to guanine substitution mutation in the second base. These mutations were not detected in 7 follicular adenoma and 1 papillary carcinoma. The results were confirmed by both dot blot hybridization and direct sequencing. Mutation-specific amplification is a powerful method for detection of known mutations of protooncogenes or tumor suppressor genes in investigation of the cancer biology.