Human megakarocyte colonies were grown from the bone marrow in plasma clot or methyl cellulose cultures. Maturation of the megakaryocytic cells was sequentially studied from day 5 to day 16 of culture by fluorescent labelling with a panel of monoclonal and polyclonal antibodies against different platelet glycoproteins (Gp), P1 A1 antigen, factor VIII RAg platelet factor 4 (PF 4), fibrinogen and platelet-derived growth factor (PDGF). Expression of Gp Ib was also studied by immunogold technique at electron microscopy. The first cells identifiable by these antibodies were found at day 5 of culture. They had the size of a lymphocyte. These small megakaryocyte precursors already expressed all the platelet antigens, HLA-DR and transferrin receptors and were devoid of erythroid or myeloid markers. Among the platelet antigens, Gp IIIa was the most sensitive marker for the identification of these precursors. However, double-fluorescent labelling demonstrated that the different platelet markers were coexpressed in a large majority of cells. Interestingly, cytoplasmic markers demonstrated that these small megakaryocyte precursors were themselves heterogenous by morphological criteria. During maturation, expression of Gps, particularly of Gp Ib, increased while the labelling pattern of anti factor VIII RAg and anti PF 4 antibodies switched from diffuse to granular staining. PDGF could also be detected in the megakaryocytes grown in culture.