The relation between the bleeding time and the megakaryocyte nuclear DNA content and size was evaluated in eleven consecutive patients with normal steady state thrombopoiesis undergoing thoracotomy. A statistically significant inverse correlation was found between the bleeding time and both megakaryocyte DNA content (r = -0.71, p less than 0.05), megakaryocyte total size (r = -0.58, p less than 0.05), megakaryocyte cytoplasmic size (r = -0.64, p less than 0.05) and megakaryocyte nuclear size (r = -0.58, p less than 0.05). The megakaryocyte total size and the megakaryocyte cytoplasmic size were statistically significantly larger in men than women (p less than 0.02 and p less than 0.03 respectively). Changes in the megakaryocytes in the bone marrow are associated with changes in primary haemostasis in normal individuals.