1. Megakaryocytes can be demonstrated with great frequency in the viscera.
2. Megakaryocytes were present in the lungs in all of 50 autopsies studied.
3. They were next most frequently found in the spleen.
4. The simultaneous occurrence in the various organs, in the absence of extra-medullary hematopoiesis, is roughly related to the concentration in the lungs.
5. Under normal conditions megakaryocytes in small numbers circulate in the blood stream.