The beta o-thalassaemia gene of an individual who was a mixed heterozygote for this allele (GAA to TAA in codon 121) and beta(+)-thalassaemia (IVS-1 position 110 G to A) was examined to determine if the beta o-thalassaemia allele directed the synthesis of any detectable protein product. This beta o-thalassaemia allele was of particular interest, because it is the only example of a premature chain termination codon in the third exon of the beta-globin gene that produces thalassaemia. A very small amount of an abnormal protein was found in the red blood cells of the proband and was purified by preparative column chromatography. This abnormal protein was digested with trypsin, the peptides were separated by reverse phase high performance liquid chromatography (HPLC), and the amino acid content of each peptide was determined. All of the soluble beta-globin peptides were found except for T-13, T-14 and T-15 (residues 121-146), indicating the presence of a truncated protein that corresponded to the translation product of the beta 121 Glu----Term mRNA. This truncated globin was estimated to comprise between 0.05% and 0.1% of the total non-alpha-globin protein. These results may explain the phenotype of inclusion body beta-thalassaemia in heterozygotes, which is atypical of heterozygous beta o-thalassaemia.