The bone marrow biopsy specimens of 35 patients with benign and malignant erythroid hyperplasias were examined for the presence of hemoglobin A, hemoglobin F, muramidase (lysozyme), and transferrin, using an indirect immunoperoxidase method (PAP) on Zenker's-fixed paraffin-embedded bone marrow biopsy specimens and particles. Five cases of each of the following entities were studied: erythroleukemia and erythremic myelosis, acute granulocytic leukemia with maturation (FAB M2), polycythemia rubra vera, myeloproliferative syndrome in childhood, megaloblastic anemia (B12 and folate deficiency), erythroid hyperplasia (regenerating bone marrow and hemolytic anemia), and Ph' chromosome positive chronic granulocytic leukemia. Hemoglobin A was present in both the early and late erythroid precursors in all conditions. Hemoglobin F was the predominant hemoglobin in early erythroblasts of pernicious anemia and in both early and late erythroid elements in erythroleukemia and erythremic myelosis. Small quantities of hemoglobin F were present in a few isolated clusters in other conditions. Staining for hemoglobin F may be useful in identifying immature erythroid precursors and in distinguishing some cases of dysplastic erythroid hyperplasia from neoplasia. Additionally, these findings suggest that the maturational switch in hemoglobin synthesis operates with distinct pathways under different conditions.