Immunodeficient athymic mice with human tumor xenografts provide an important in vivo experimental model for cancer research. However, only a limited number of tumor types grow in these animals. For human breast carcinomas, the incidence of tumor-take is 6-15%. Recently, increased incidence of xenograft development in mice has been reported for various human tumors when the tumors were coinjected with Matrigel. We studied the development of human breast carcinoma xenografts in athymic mice with and without coinjection of Matrigel. Tumors developed in only 7.3% of enzyme-dispersed tumors injected subcutaneously in saline solution alone. None of these tumors metastasized to distant sites. On the other hand, 50% of enzyme-dispersed tumors coinjected with Matrigel developed xenografts; four out of five of these tumors metastasized to distant sites. Our data from the recent study suggest that, in athymic mice, Matrigel not only enhanced breast tumor growth but also facilitated tumor metastasis.