FOR the past several years, Ehrlich ascites tumour cells have been used in this Laboratory in work on the anti-tumour activity of fatty acids1–3. Serial transplantation of the stock tumour in Swiss mice was carried out at weekly intervals using an inoculum of 15 × 106 washed ascites cells, a number which effected a maximum rate of cell proliferation4. Under these conditions, the proportion of tumour cells in the ascitic fluid reached 60–70 per cent within 5–6 days after inoculation. Afterwards, however, the percentage of tumour cells declined steadily and dropped to only 10–20 per cent shortly before death of the mice at 10–14 days. This observation suggested that the ascitic fluid might be exerting an oncolytic effect on the tumour cells5,6, and experiments were carried out to test the activity of the fluid after removal of the tumour cells by high-speed centrifugation. These experiments, rather than revealing an oncolytic factor, indicated the presence of a transmissible agent producing ascites tumours.