The diagnostic potential of immunocytochemical investigation of human bone marrow has not been fully realized due to difficulties in morphological identifying of immunostained cells. We used an indirect immunoperoxidase technique after May-Grünwald-Giemsa staining for simultaneous morphological and immunocytochemical analysis of blasts in human bone marrow. Six healthy bone marrow donors were investigated. Most blasts I expressed CD34, CD38 and HLA-DR. Expression of c-kit (CD117) was observed on 42 +/- 9% of blasts I. Granulocytomonocytopoietic character was demonstrated by expression of CD13 (33 +/- 15%) and CD45RA (23 +/- 10%) and erythropoietic character was demonstrated by expression of CD36 (22 +/- 8%) and CD45RO (30 +/- 11%). A very low proportion of blasts I were Thy-1 and CD61 positive; 34 +/- 6% of blasts I expressed CD22, representing B lymphoid committed progenitors. CD3, CD15, and glycophorin A expression was not detected. Blasts II and III and proerythroblasts did not show CD34 positivity. We conclude that blasts I are morphologically identifiable cells with a high percentage of CD34, CD38, and HLA-DR positivity. They are a pool of committed progenitor cells for erythropoiesis, granulocytomonocytopoiesis, megakaryocytopoiesis, and B cell development. Blast II and proerythroblast represent the first morphologically identifiable cells of granulocytopoiesis and erythropoiesis.