Summary.-The growth of carcinoma of the human large bowel was studied in the first 2 passages in immune deprived mice. The tumours were obtained from large bowel resections on 3 people. There was a strong histological similarity between the patient's tumour and the tumour that grew subcutaneously in the mice 2-8 months after implantation. One dissimilarity observed was a higher mitotic index in some of the tumours growing in the immune deprived animals. In the second passage of the bowel tumours, cells were implanted into groups of 8-10 animals in the following sites: subcutaneous, intramuscular, intravenous, intrahepatic, intraperitoneal and intrathoracic. Growth of tumour was observed from all 3 tumours when they were implanted subcutaneously, intramuscularly, intraperitoneally and intrathoracically. Infiltration of muscle by tumour was a frequent finding. Lung metastases developed after intravenous injection of cells in 1 of the 3 tumours. In none of the 3 tumours did growth follow injection of cells directly into the substance of the liver. On no occasions were spontaneous metastases observed.