The authors tested the ability of scattered tumor cells to re-form a tumor in vivo. Disseminated tumor cells are morphologically visible stained with May-Grünwald Giemsa in the lung, liver, kidney, and spleen of Yoshida ascites tumor bearing rats. Free tumor cells can easily be fine needle aspirated from those organs and injected in syngeneic Wistar rats. All the host rats show ascites tumor take after intraperitoneal transplantation of each aspirated sample. This biological model might be useful to study in vivo a wide range of properties of neoplastic and non-neoplastic host cells.