The coital mode of transmission of the transmissible veneral tumour (TVT) of the dog is well established, but the mechanism of transmission of this neoplasm is uncertain. Immunological studies were carried out in order to investigate this problem. Using the indirect membrane immunofluorescence test, it could be shown that the isoantigens of the host cannot be detected on the cell surface of the TVT with isoimmune sera sera directed against host antigens. Furthermore, it was determined that anti-TVT sera sera contain isoantibody activity. These findings suggest that the TVT is transmitted from one dog to another by the transplantation of living cells and not by an oncogenic agent which transforms normal host cells.