Erythrocyte volume and haemoglobin concentration in individual red cells from 62 patients with Hb H disease: 37 with H genotype (α-thalassaemia 1/α-thalassaemia 2) and 25 H/CS genotype (α-thalassaemia 1/Hb Constant Spring) were measured using the H*l haematology analyser. All 25 cases with H/CS genotype, the more severe genotype, had microcytes (red cells with a volume smaller than 60 fl) less than 35% and hypochromic red cells (red cells with haemoglobin concentration less than 28 g/dl) more than 35%. A discriminant function, the ratio between the percentage of hypochromic red cells and the percentage of microcytes (Hypo/Micro), was proposed. Most of the H/CS patients (76%) had Hypo/Micro greater than 2.5 whereas those of H patients (82%) were below 2.5. Red cell volume histograms were also characteristically different between the two genotypes: the H/CS had a peak between 60 and 90 fl while the H genotype showed a peak at or very close to 60 fl, indicating a greater degree of microcytosis. Increased hypochromia with a slight decrease in cell size of H/CS red cells suggests that the poor degree of haemoglobinization has no linkage or very little role in disturbing the synthesis of membrane proteins and their assembly to the plasma membrane.