We have conducted serial studies on the role of matrix metalloproteinases (MMPs), especially MMP-9, in tumor invasion and metastasis. In 9 human carcinoma cell lines derived from lung, prostate and melanoma, we found, by zymography and Western blot, that the expression levels of MMP-2 and MMP-9 correlated well with their invasive as well as metastatic abilities both in vitro and in nude mice. When anti-sense MMP-9 cDNA was introduced into WM451, a highly metastatic human melanoma cell line with high expression level of MMP-9, a significant down-regulation of MMP-9 protein expression was found. Meanwhile, the number of cells passing through Matrigel-coated membrane (in vitro invasion assay) and spontaneous metastases to lymph nodes and lungs were significantly reduced. Furthermore, when tissue inhibitors of metalloproteinases-1, -2 or -3 (TIMP-1, TIMP-2 or TIMP-3) cDNAs were individually transtected into metastatic cancer cells, remarkable inhibition of invasion and metastasis were also noticed in each group. These results demonstrate that either up-regulation of TIMPs or down-regulation of MMPs could significantly inhibit the expression of malignant phenotypes, suggesting the important role MMP-9 plays in tumor invasion and metastasis.