In the course of the study of megakaryocytes of mice with grafted tumors (Sarcoma 180), sections of the lungs, liver, adrenals and lymphatic nodes were observed. This study resulted in the following: 1. A correlation was found between the occurrence of these cells in the lungs and the medullary megakaryocytopoiesis. 2. Under pathologic conditions (neoplasms) which stimulate megakaryocytopoiesis, the numbers of pulmonary megakaryocytes were increased. In the inoculated mice, the more common among the megakaryocytes were the largest forms with multilobulated nucleous mostly piknotic. 4. Megakaryocytes were observed frequently in the liver of mice with Sarcoma 180.