Three formulae have been advocated for interpreting the blood-count when differentiating between iron deficiency and heterozygous thalassaemia: the discriminant function. M.C.V.-R.B.C.-(5 X Hb)-k, the M.C.V/R.B.C. ratio, and the (M.C.V.)2 X M.C.H. product. In a comparison of their diagnostic accuracy in microcytic adults from several countries, the discriminant function was the most satisfactory. 417 of 455 uncomplicated cases (91.6%) of iron deficiency, heterozygous alpha1- and beta- thalassaemia were correctly identified. The function was not able to distinguish heterozygous alpha2-thalassaemia from iron deficiency. A flow chart illustrates how the discriminant function can be used in haematological practice.