The fact that human tumor xenografts grown in immunodeficient mice have proven to be useful models of human cancer is well documented. However, the establishment of such xenografts from cell lines cultured in vitro has proven to be fraught with difficulties-these problems become even more apparent when one tries to establish xenografts from primary clinical material (1 -3 ). Nevertheless, the inclusion of Matrigel has been shown to enhance the tumorigenicity of a number of cell lines in vivo (4 -10 ), including those of ovarian origin (11 ). There are also reports of Matrigel enhancing the tumorigenic potential of breast primary material (12 ). Furthermore, it has been shown that xenografts established in the presence of Matrigel can be excised, transferred into recipient animals, and subsequently grown in the absence of Matrigel, which suggests that it is required only for the initial establishment of tumors (11 ). Matrigel is, therefore, extremely useful for establishing a variety of cell lines as routine xenografts, which could otherwise prove difficult to take, as illustrated in Table 1 and Fig. 1 (adapted from [11 ]).