Five lines of human breast-carcinoma xenografts have been tested for sensitivity to cyclophosphamide, methotrexate, 5-fluorouracil, adriamycin, vincristine and melphalan, alone and in combination, using tumour growth delay as an end-point. The xenograft lines were established and passaged in mice immune-suppressed by thymectomy and whole-body irradiation. There was a considerable range of sensitivity of the different lines to the agents studied, and within this variation there was evidence that the most effective single agent or combination differed for each tumour. Combination chemotherapy was more effective than single agents in 3 of the lines, but melphalan was more effective than either combination in the other 2. It is suggested that a panel of human breast tumours grown in immune-suppressed mice may prove useful in testing new cytotoxic agents for activity against breast cancer before their use in clinical trials, and that more effective combinations of existing drugs might be designed with the aid of this system.