The proportion of Hb A2 was 3.53 +/- 0.94 per cent in the blood of 69 persons with sickle cell trait (A/S) and 2.88 +/- 0.69 per cent in a like number with a normal (A/A) genotype using a radial immunodiffusion assay. This differs significantly at the 0.1% level by the paired comparison technique. It is proposed that this effect is due to the impaired association of beta S with alphaA chains allowing more of the latter to combine with delta-chains. A similar phenomenon may account for the increased proportion of Hb A2 observed in beta-thalassemia. The proportion of Hb F did not differ in the two genotypes.