Analyses of DNA from 64 patients with thalassemia major using the hybridization technique of amplified DNA with radiolabeled synthetic oligonucleotide probes identified 13 different beta-thalassemia mutations. The codon 39 (C----T) and IVS-I-110 (G----A) mutations occurred most frequently but seven additional mutations were observed which were present at frequencies of 3.9 to 10.2%. This broad spectrum of beta-thalassemia alleles complicates the analyses for institutions involved in prenatal diagnosis. Promoter mutations were rare and the frequencies of two other mild mutations [IVS-I-6 (T----C) and the poly A mutation] were relatively low indicating that beta-thalassemia is a severe disease among Bulgarians. The high frequencies of 4.7-5.5% for the four frameshifts at codons 5, 6, 8, and 8/9 may be specific for this population.