Forty-five human malignant tumour specimens were cultivated in vitro in an attempt to determine the necessary conditions for tumour cell maintenance and to establish permanently-growing cell lines. Continuously-growing cultures were derived from five tumours, including carcinomas of the oesophagus and colon, a hepatoma, a mesothelioma and a retroperitoneal sarcoma. The carcinoma of the oesophagus and the hepatoma, which have adapted fully to in vitro conditions, can be regarded as established cell lines.