Hemoglobin Constant Spring (Hb CS) is a hemoglobin variant with an elongated alpha-globin chain secondary to a chain termination mutation. The diagnosis of HbCS by electrophoresis is difficult because it is present in very low amounts in the red cells of heterozygotes. Selective enzymatic amplification of the alpha 2-globin gene and allele-specific hybridization for Hb CS gene provided accurate diagnosis of Hb Constant Spring. We have used this approach to detect the alpha cs mutation in the cord blood that contained all four alpha-globin genes but had Hb Bart on electrophoresis. The alpha cs mutation was found in six subjects whose Hb Bart levels were 3.0, 3.2, 3.7, 4.0, 4.9, and 9.8%. The latter also had -alpha mutation on the other chromosome, giving rise to the genotype alpha cs alpha/-alpha, which produced high Hb Bart. The gene frequency for alpha cs in the Thai calculated from a total of 406 cord blood studied in Bangkok was found to be approximately 0.008.