Increased levels of hemoglobin A(2) (HbA(2)) are present in most beta-thalassemia carriers. The mechanism of this effect is not understood, although the increase may result from transcriptional and posttranscriptional changes. In the present study, we quantitate delta-globin mRNA levels in peripheral-blood-enriched reticulocytes and characterize the variation of delta-mRNA levels in 30 beta-thalassemia heterozygotes who individually carry one of the four common Chinese beta-thalassemia alleles [codons 41/42 (-TTCT); codon 17 (A-->T); IVS-II-654 (C-->T); -28 (A-->G)]. A sensitive and quantitative competitive reverse-transcriptase polymerase chain reaction method was developed and used to assess the absolute amounts of delta-mRNA transcripts in these peripheral erythroid cells. The results showed a large increase in delta-mRNA amounts in all the carriers examined (72.3 +/- 9.0 amol/microg RNA) as compared with those in 12 controls (1.2 +/- 0.2 amol/ microg RNA). There was a direct correlation between the delta-mRNA levels and types of beta-thalassemia alleles; generally, the delta-mRNA levels are higher in heterozygotes for beta(0)-thalassemia mutations than beta(+)-thalassemia mutations. The delta-mRNA levels correlated inversely with hemoglobin and red cell indices but directly with HbA(2) levels in heterozygotes of each of the group of beta-thalassemia mutations. These results suggest that a greater impairment in beta-globin gene expression results in increased transcription of delta-globin gene and in a higher level of HbA(2).