Eight serially transplantable human breast-cancer xenograft lines have been established in immune-suppressed mice. Specimens from 102 primary and secondary lesions obtained at surgery from 80 patients were implanted into mice immune-suppressed by thymectomy and whole-body irradiation. A number of variations of implantation site, transplantation technique, method of immune suppression and hormonal manipulation of the host were tried in an attempt to increase the take rate, but without success. The 8 lines established have been serially transplanted into further immune-suppressed mice for at least 2 passages, and appear to maintain characteristic human histopathology, chromosome number and tumour-marker production. None of the tumours show hormone sensitivity. The poor take rate may be a reflection of the biological nature of breast cancer rather than a failure of the immune-deprivation technique, as many other human tumours grow well as xenografts in this system.