A trimodal distribution of the amounts of Hb S in 56 sickle cell heterozygotes was observed. The variable amounts of Hb S were directly related to the mean cell volume and the mean cell Hb and were a result of the difference in net synthesis of Hb S due to the presence of the alpha-thalassaemia. Double heterozygotes for the Hb S and alpha-thalassaemia-1 genes, besides having a lower amount of Hb S in the red blood cells, were found to have lower Hb and slightly elevated Hb A2 levels.