Pure megakaryocytic colonies, megakaryocytic-erythroid colonies and mixed hemopoietic colonies can be cultured from human bone marrow under appropriate culture conditions. Human plasma and mercaptoethanol support the growth for these different types of hemopoietic colonies. However, the addition of medium conditioned by leucocytes in the presence of phytohemagglutinin (PHA-LCM), as a source of thrombopoietin, is required for the formation of megakaryocytic colonies or megakaryocytes within mixed colonies. Megakaryocytes were identified by their typical morphological appearance in culture. Pure megakaryocytic colonies, megakaryocytic-erythroid colonies and mixed colonies were plucked by micropipette and analysed by the PAP-slide technique using antibodies to human factor VIII-related protein or serum derived from a patient with posttransfusion purpura; this particular serum demonstrated anti-P1A1 antibody activity. These antibodies might provide an excellent probe to identify megakaryocytic progeny from committed and non-committed hemopoietic progenitors, facilitating studies of early events in megakaryopoiesis.