Athymic nude mice have been used in recent years to study the biology of human tumors and to assess therapeutic responses in vivo rather than just in vitro. Some human tumors metastasize in nude mice, providing model systems for analyzing various aspects of the metastatic phenotype of human neoplasms. For breast carcinomas, however, the tumor-take rate of surgical specimens is low, and only a limited number of cell lines proliferate in nude mice. The site of injection of the breast carcinoma cells is important; tumors grow at a lower inoculum dose and with shorter latent intervals after implantation in the mammary fatpad of nude mice than after injection in the subcutis. One breast carcinoma cell line, MDA-MB-435, metastasizes from mammary fatpad tumors to lymph nodes, lungs, and other visceral organs. In contrast, two other cell lines show lower metastatic ability. Intravenous injection and injection of tumor cells into the internal carotid artery of nude mice produces lung and brain metastases, respectively, thus simulating the arrest and organ colonizing steps of the metastatic cascade. These different techniques demonstrate the potential of experimental studies of human breast cancer growth and metastasis using nude mice.