A rise in the blood platelet count and volume occurred in 11 patients following traumatic splenectomy. The pattern of the volume distribution, however, remained unchanged. This indicates an increased rate of platelet production, with the newly formed cells substantially larger than their preoperative counterparts, and can be interpreted in terms of a humoral factor of splenic origin that is able to exert its control over the bone marrow. The stable shape of the volume distribution, despite splenectomy, suggests the presence of an organ other than the spleen that is capable of maintaining the normal age distribution in the peripheral circulation, and the liver is proposed as a possible candidate for this role. The evidence, so far, has been circumstantial and further investigation will be required before definite conclusions can be drawn.