Blood nucleated cells collected by leukapheresis and spleen cell suspension from patients with myelofibrosis with myeloid metaplasia (MMM) were studied for their haematopoietic capacity. Using committed progenitor cell assays (CFU-GM, BFU-e) and a one-stage long-term liquid stem cell system, we have shown: (1) a preferential expansion of the circulating committed progenitor cell pool above the more primitive stem cell compartment; (2) the absence of any development of a stromal adherent layer in long-term cultures of peripheral blood nucleated cells suggesting the self-sustaining capacity of the circulating primitive stem cells; (3) that the spleen is only a production site of committed progenitor cells but does not generate primitive stem cells; (4) the presence, in the spleen, of stromal progenitor cells. We conclude that the peripheral blood primitive stem cells in patients with MMM are not of splenic origin.