Abstract: In vitro megakaryocytopoiesis was studied in 8 patients with chronic immune thrombocytopenia (ITP). A significant increase of megakaryocyte (MK) colony formation was observed in 5/5 patients studied. Furthermore, the serum of these 8 patients was able to enhance MK colony formation by normal marrow cells. This effect was neither due to a decrease of inhibitors of megakaryocytopoiesis such as betathromboglobulin ((3TG) nor to the IgG fraction of patients' serum. In addition, the level of interleukin 6, which is above all a stimulus for MK maturation, was found within the normal range in 8/8 patients tested. These data suggest that in chronic ITP there is an increase of MK progenitor cell number which may be due to an increased level of MK colony-stimulating activity.