A new cell line Rat mammary (Rama) 900 was isolated from the ascitic version of the SMT-2A metastasising rat mammary tumour by stepwise adaptation of the tumour cells to tissue culture. The cells grew mainly as loosely-adherent aggregates, and were dependent during the first 18 passages in vitro on a feeder layer of mesothelial-like cells (Rama 950) obtained from the same tumour. Subcutaneous injection of Rama 900 cells in fat pads of syngeneic Wistar Furth rats yielded anaplastic primary tumours and extensive, gross metastases including those in lungs, lymph nodes, liver and bones, similar to the parental transplantable tumour. The extent of metastatic spread from subcutaneous fat pads was increased by passage 17 in vitro for the Rama 900 cells. A similar extent of metastatic spread was achieved at earlier times by injecting the original cells with the non-tumorigenic Rama 950 cells in vivo. Subcutaneous injection of Rama 900 into thymectomised rats or MF1 nu/nu mice yielded fewer tumours, most of which regressed. No metastases occurred in the thymectomised rats and fewer metastases, mainly in lungs but not in lymph nodes, livers or bones, were seen in the nude mice. The ascitic tumours formed by intraperitoneal injection of nude mice contained both anaplastic rat cells similar to Rama 900 and mouse mesothelial-like cells similar to Rama 950. Although these anaplastic ascites cells failed to yield any tumours in syngeneic or thymectomised rats, they still produced tumours and metastases, including those in lymph nodes, in nude mice.
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