Matrix metalloproteinases (MMPs) are thought to play a key role in tumor invasion and metastasis. The role of MMP-9 (gelatinase B) in tumor metastasis was examined in MMP-9-deficient mice produced by gene targeting using embryonic stem cells. MMP-9-deficient mice develop normally and are fertile. In these mice, the number of metastatic colonies of B16-BL6 melanoma cells or Lewis lung carcinoma cells that were implanted intravenously fell by 45% for B16-BL6 melanoma and 59% for Lewis lung carcinoma (p = 0.03 and p = 0.0043, respectively). Gelatin zymography showed that both tumor cell lines did not secrete MMP-9 by themselves but the host cells surrounding the tumor cells secrete MMP-9 in vivo. These results indicated that host-derived MMP-9 plays an important role in the process of tumor metastasis.