Myelofibrosis is the most representative model of the association between the hemopoiesis failure end the abnormalities of the myeloid microenvironment. Long term culture in Dexter's liquid system is a good model for the in vitro study of myeloid differentiation. An experimental in vitro system using an irradiated under layer and aplastic patients plasma has yielded some preliminary results allowing the production of megakaryocytic colonies. Close contact between these colonies and the fibroblasts or stromal cells in the adherent layer could probably be used for the study of the interrelation between the stromal abnormalities and myeloid pathology.