The tissue inhibitors of metalloproteinases (TIMPs) are natural inhibitors of the matrix metalloproteinases, a group of zinc-binding endopeptidases involved in the degradation of the extracellular matrix. We have isolated overlapping cDNAs encoding a novel human TIMP, TIMP-3. The cDNAs contain a 591-bp-long open reading frame encoding 9 amino acid residues of the signal peptide and 188 residues of the mature TIMP-3 polypeptide. Both the nucleotide sequence and the conceptual translation product of the human TIMP3 cDNA have a high degree of similarity to ChIMP-3, a recently cloned metalloproteinase inhibitor in the chicken, and to the TIMP1 and TIMP2 gene products, including 12 conserved cysteinyl residues at the same relative positions. The TIMP3 gene is expressed in many tissues, with highest expression in the placenta. Upon hybridization with a panel of human-hamster somatic cell hybrid DNAs, the TIMP3 gene segregated with clones containing chromosome 22. Using in situ hybridization to human metaphase chromosomes, we have assigned the locus for the TIMP3 gene to the q12.1-q13.2 region of human chromosome 22.