The value of serum ferritin and mean corpuscular volume (MCV) measurement in distinguishing between iron deficient, β -thalassaemia trait and normal subjects has been studied. Normal subjects had normal ferritin and MCV, iron-deficient ones had low ferritin and low or normal MCV, and thalassaemics had normal ferritin and low MCV. By the combined use of these two measurements it was possible to identify individuals belonging to one of the three categories with an accuracy of over 95%. Although definitive diagnosis of β-thalassaemia trait requires the demonstration of abnormal haemoglobin A2 levels or reduced β-ehain synthesis, serum ferritin and MCV measurements are useful screening procedures for the initial diagnosis of β-thalassaemia trait and iron deficiency. Because of the very small amounts of blood required for both of these measurements, they are particularly suitable for surveying large numbers of subjects in populations with a high prevalence of hypochromic-microcytic anaemias.