Intracellular hemoglobin represents an excellent marker for specific characterization of normal, megaloblastic, or dysplastic erythroid cells in paraffin sections. Using an immunoperoxidase indirect sandwich technique for detection of intracellular hemoglobin, erythroid cells at all stages of maturation were readily identified in bone marrow biopsies (58 specimens total) with a) normal erythropoiesis, b) megalobastic erythropoiesis, and c) various myeloproliferative disorders, including erythroleukemia. In other tissues (6 spleens, 2 lymph nodes, 1 liver) with extramedullary hematopoiesis, erythroid cells were similarly defined on the basis of this immunohistochemical method. Initial fixation in Zenker's-acetic acid solution (employed for bone marrow biopsies), B5 solution, or formalin, appeared equally effective in preserving the antigenicity of intracellular human hemoglobin. This sensitive and specific immunoperoxidase technique for erythroid cell characterization is particularly applicable to tissues with abnormal erythropoiesis, in which precise cell identification generally presents a diagnostic problem.