Among patients with ITP, a group of 9 subjects displayed a reduced platelet turnover. In their bone marrow the megakaryocyte mass was measured by 59Fe, and the results were referred to those of 6 patients with a moderately increased and of 6 with a considerably increased platelet turnover. In the low-turnover group the megakaryocyte nuclear lobe number was subnormal cells with basophil cytoplasm were in the majority and the number of megakaryocytes was increased, whereas in the other two groups of megakaryocite population showed a shift to the right, and increased nuclear lobe number, and the number of megakaryocytes, related to the erythroid cells, was increased. It is therefore assumed that in the case of a reduced platelet turnover, the megakaryocytes are damaged, primarily their maturation in the bone marrow. The three groups showed, however, no clinical differences.