In vitro DNA amplification and dot blot analysis with synthetic allele specific oligonucleotides (ASO) identified the beta + IVS, I-6 (T --> C) thalassaemia in 78% of 32 chromosomes from 16 beta-thalassaemia homozygotes in Malta. The preponderance of a single thalassaemia mutation in one population is unusual. The beta + IVS, I-6C thalassaemia mutation was also found in three carriers who had an associated beta globin heterozygosity, i.e. Hb Valletta (or alpha 2 beta 2 87PRO) or Hb S (or alpha 2 beta 2 6VAL). The proportion of Hb A in these cases (av. = 29.7%) provided objective documentation of the relatively mild effect of this mutation on in vivo globin gene expression. However, the expression of homozygous disease was more severe in developing children compared to adults. The beta + IVS, I-6C mutation complicates population testing because heterozygotes can have Hb A2 levels below those classically associated with beta thalassaemia.