We present in vivo evidence that there is no reduction in beta-mRNA accumulation in patients with nonsense codons in the terminal exon of the beta-globin gene. Using reverse transcriptase/polymerase chain reaction (RT-PCR), beta-globin cDNA was isolated from the reticulocytes of individuals heterozygous for nonsense codon mutations in exons II and III of the beta-globin gene. Clinically asymptomatic individuals heterozygous for mutations causing premature termination of translation in exon II [beta(0)39(C-T) and F/S71/72(+A)] were found to have almost no mutant beta-cDNA, whereas patients with nonsense codon mutations in exon III [beta 121(G-T) and beta 127(C-T)] with the clinical phenotype of thalassemia intermedia had comparable levels of mutant and normal beta-cDNA. Translation of the mutant beta-mRNA from patients with nonsense codon mutations in exon III would give rise to truncated beta-globin chains, which could explain the more severe phenotype seen in these individuals.