A quantitative and qualitative study of megakaryocytes of bone marrow in mice, inoculated intraperitoneally with "Sarcoma 180", showed the following results: 1. It was noticed a real increase in the total number of megakaryocytes during the neoplastic process. 2. This increase was cosnpicuous enough within the first 24 hours. 3. The megakaryocytes stabilized in their highest main values, after the third day. 4. megakaryocytes were represented mostly in large forms, sometimes producing clusters. 5. It has been made a parallel confront between the results found in mice inoculated intraperitoneally and those ones found in mice inoculated intravenously.