The clinical usefulness of the measurement of red cell zinc protoporphyrin (ZPP), an indicator of iron-deficient erythropoiesis, was assessed in a group of UK children undergoing investigation for red cell microcytosis. Of 213 children studied, 136 had increased ZPP values. Of these, 86 also had reduced iron stores as indicated by serum ferritin concentration. The 50 children with increased ZPP and normal ferritin values could be divided into two main groups. One group comprised 28 children who had evidence of coexistent infection or inflammatory disease. The other included 21 children who had beta-thalassemia trait (n = 19) or disease (n = 2). Among the 77 children with normal ZPP values, 22 had reduced serum ferritin concentrations and 45 did not, nor did they have evidence of beta-thalassemia. Microcytosis in some of these children could have been due to alpha-thalassemia trait. Measurement of ZPP is a simple, quick, and relatively cheap method of confirming the presence of iron-deficient erythropoiesis even when inflammation makes serum ferritin measurements unreliable. It is not as sensitive as the ferritin assay to the early stages of iron deficiency, and its specificity is reduced by the occurrence of raised values in most children with beta-thalassemia trait. Where there is microcytosis, normal values, together with normal hemoglobin A2 and serum ferritin concentrations, are likely to indicate alpha-thalassemia trait.