A partial cDNA encoding the 3' end of a putative novel human matrix metalloproteinase (MMP) was identified by sequence similarity searching of databases containing expressed sequence tags. The remaining 5' end of the MMP cDNA was amplified by PCR from human mammary gland cDNA. The predicted protein product displays all the structural features characteristic of the MMP family and has closest identity with MMP-1, -3, -10, and 11. We have provisionally designated this novel MMP as MMP-18. MMP-18 mRNA is expressed in a wide variety of normal human tissues, including mammary gland, placenta, lung, pancreas, ovary, small intestine, spleen, thymus, prostate, testis, colon, and heart, but is not detected in brain, skeletal muscle, kidney, liver, or peripheral blood leucocytes.