Uncontrolled expression of matrix metalloproteinases 2, 3 and 9 (MMP-2, -3 and -9) is believed to be a critical part of the invasive potential of tumor cells because of their ability to degrade type IV collagen, a major structural component of basement membranes. Availability of proteolytic activity in the vicinity of the cell surface is further affected by a local balance between the enzymes and their inhibitors produced by the cell. To determine how frequently deregulated expression of the MMPs and tissue inhibitors of metalloproteinases (TIMPs) is associated with tumor cells, 26 human tumor cell lines were examined by Northern blotting. Transcripts for MMP-2 and MMP-9 were more frequently expressed in mesenchymal tumor cells (9/9 for MMP-2 and 6/9 for MMP-9) than in epithelial tumor cells (4/17 for MMP-2 and 2/17 for MMP-9). Although expression of MMP-2 mRNA was clearly cell type-specific, MMP-9 mRNA expression in mesenchymal cells correlated well with the reported tumorigenicity of the cells. Enhanced expression of MMP-9 mRNA was also associated with the tumorigenic transformation of cells by an activated c-H-ras gene in human embryonic fibroblasts. Only 3 of the 26 tumor cells expressed MMP-3 mRNA, and 2 of the 3 were epithelial tumor cells which coordinately expressed MMP-9 and TIMP-1 mRNAs. TIMP-1 mRNA was almost undetectable in 50% of the tumor cells, but TIMP-2 mRNA was expressed in the majority of the cells. These findings provide comprehensive information about mRNA expression of the MMPs and TIMPs in tumor cells, the deregulation of which is thought to be an integral part of the invasive potential of tumor cells.