Two conditions are liable to lower the α:β globin biosynthesis ratio in reticulocytes: iron deficiency and α-thalassaemia. The present paper studies the effect of haemin on reticulocytes from 12 patients who have α-thalassaemia and/or are iron deficient. The α:β globin biosynthesis ratio was improved in all these cases. 4 showed initially an α:β synthesis ratio usually associated with α-thalassaemia type-1; on the addition of haemin the ratio rose to that associated with α-thalassaemia type-2. In the other 8 patients the ratio was initially typical for α-thalassaemia type-2, and on addition of haemin the ratio became normal. It is suggested that in iron deficient patients a diagnosis of α-thalassaemia type-1 or type-2 cannot be made unless haemin has been added to the test system. If this is not done iron deficiency alone can cause the α:β globin synthesis ratio to resemble that associated with α-thalassaemia tyρe-2, and iron deficiency in combination with α-thalassaemia type-2 can cause the ratio to resemble that typical for α-thalassaemia type-1. Reticulocytes from 8 α-thalassaemic patients without iron deficiency did not show a marked haemin effect «5%), and in 1 patient with iron overload, the ratio actually fell by about 10%.