Characteristics of circulating CFU‐MK and the effect of serum and plasma on CFU‐MK growth were studied in 14 patients with primary myelofibrosis (MF) using short‐ and long‐term culture methods. The number of CFU‐MK in short‐term cultures was significantly increased in the non‐splenectomized patient group (p<0.01). Without added PHA‐LCM and normal serum, spontaneous colony formation was found in 9 out of 10 patients. In long‐term blood cultures from 6 MF patients, 3 untreated patients formed confluent adherent layers and produced in suspension an equal number or an even greater number of nucleated cells, megakaryocytes and CFU‐MK than those obtained in long‐term bone marrow culture from normal individuals. 2 splenectomized patients showed neither an increased numbers of CFU‐MK nor the capacity to develop an adherent layer. The serum and plasma of MF patients failed to stimulate megakaryocyte colony formation by normal bone marrow in a normal fashion. These findings indicate a megakaryocytopoietic abnormality, and a central role of the spleen in extramedulla
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