Breast cancer is one of the most common sources of brain metastases. After clinical detection of such tumour spread, median survival is usually only a few months, with brain metastases being the major cause of death (Boogerd et af., 1993). Between 20 and 30% of breast-cancer patients have metastatic brain lesions at time of death (Lee, 1985). Moreover, the established modes of treatment for brain metastases give 1-year survival rates of less than 20%. Clearly there is a need to further our understanding of the biology and treatment of brain metastasis in breast cancer.It is well known that some cancers metastasise in a selective manner to organs distant from their primary tumour site. Breast-cancer metastasis to the brain is an example of this pattern of organ colonization. Our understanding of such selective metastasis has been assisted by the use of animal models, which also provide a valuable resource in developing novel forms of treatment. In the establishment of such models, the nude mouse has proved to be an excellent host for many human tumour xcnografts (Fodstad, 1991), while maintaining good conservation of histological and phenotypic characteristics of the original tumour (Fidler, 1986). However, breast tumours and breast-derived cell lines remain among the most exacting to establish and transplant successfully into cxpcrimental animals (Engel and Young, 1978;Fodstad, 1991;Murthy et al., 1995). Indeed. the transplantability or "take rate" of breast tumours is estimated to be less than 10% (Fodstad, 1991). Consequently. reproducible brain metastases from breast carcinomas have been demonstrated in only a few studies using an intracarotid injection route in mice (Schackert et al., 1989; Zhang et al., 1991, 1992). Although this injection model for brain metastases appears to be successful, there arc certain limitations. Moreover, the relatively small number of reports suggests that additional experimental models of brain metastasis from breast cancer are required. In the present report, we describe a reproducible model for brain metastases in nude mice using a new human breast-cancer cell linc, MA11. Compared with other studies, our experimental model seems to more closely reflect the clinical situation and, together with the relatively simple injection procedure, should favour its general application for biological and therapeutic research. MATERIAL AND METHODS PatientA 65-year-old Caucasian female attending the Department of Gynaecology, Heidelberg, underwent surgery for removal of a breast lump with axillae dissection in September 1993. Subsequent histological diagnosis confirmed a grade-I1 invasive lobular carcinoma (classic tubular-lobular type) 3 cm in diameter, and one axillary-lymph-node metastasis was found. TNM tumour staging was thus classified as Tz NI Mo (according to the UICC AJC clinical staging system). Cytological analyses of the primary tumour indicated a diploid-cell population with an S-phase level of 4.1%. The tumour was positive for the hormone receptors ER (50 pmol/g) and PgR (23 pm...