We have developed a method for distinguishing fragments of DNA that contain single-base mismatches from their perfectly paired homologues. Single-stranded regions within a duplex fragment are accessible to 1-cyclohexyl-3-(2-[4-(4-methyl)morpholinyl]ethyl)carbodiimide, which reacts with unpaired guanidylate and thymidylate residues in DNA. Intact linear duplex DNA molecules do not react with carbodiimide, whereas DNA molecules containing single-base mismatches react quantitatively. After carbodiimide reaction, the DNA molecules are electrophoresed in high-percentage polyacrylamide gels so that modified and unmodified fragments can be resolved. Application of this technique should make it possible to locate and purify DNA fragments that exhibit sequence differences from those that do not; these might be used to signal phenotypic variation as well as to diagnose inherited disease.