The formation of heteroduplexes from the amplified products of homologous alleles has been shown to be useful in the identification of heterozygotes carrying deletion or insertion mutations. Here, we describe an improved procedure that allows the detection of single base pair (bp) deletions on nondenaturing polyacrylamide gels. Carriers for a common Mediterranean beta-thalassemic mutation, beta6 (-A), could be easily detected by use of this method, as could carriers of a 1-bp deletion in the cystic fibrosis gene.