We describe a method for screening for dispersed point mutations, based on the observation that RNase frequently cleaves both strands of base pair mismatches in duplex RNA targets. The mismatched substrates are generated by in vitro transcription of wild-type and mutant templates amplified by the PCR or reverse transcription (RT)-PCR; bacteriophage promoters are incorporated into the PCR primers to permit both strands of the products to be transcribed into RNA. Complementary wild-type and mutant transcripts are hybridized and treated with RNase, and the cleavage products are separated on agarose gels and detected by visualization of the ethidium-stained sample under UV light. The method is thus non-isotopic, and since the cleavage products remain double-stranded during analysis, the labor-intensive RNase inactivation steps required in the original procedure can be eliminated. Also, nonspecific background cleavage is reduced so that longer target regions (1 kb) can be screened in a single step. The Non-Isotopic RNase Cleavage Assay (NIRCA) achieved a detection rate of 88%-90% in blind studies in a Factor IX model system, and it was also used to detect unknown p53 mutations in breast tumor samples. NIRCA provides a rapid method for sensitive, non-isotopic, high-throughput genetic screening.