A neutral protease has been extracted from the media of cultured metastatic tumor cells and purified approximately 1000 times after sequential ammonium sulfate fractionization, concanavalin A column chromatography, and molecular sieve chromatography. The protease has an apparent molecular weight of 70 000--80 000, is inactive at acid pH, requires trypsin activation, and is inhibited by ethylene-diaminetetraacetic acid but not by phenylmethanesulfonyl fluoride, N-ethylmaleimide, or soybean trypsin inhibitor. The enzyme produces specific cleavage products for both chains of pro type IV collagen isolated without pepsinization and apparently cleaves at one point in a major pepsin-extracted chain of placenta type IV collagen. The partially purified enzyme fails to significantly degrade other collagens or fibronectin under digestion conditions in which specific reaction products are produced for type IV collagen. The existence of this enzyme is significant since previously described animal collagenases fail to degrade type IV collagen. Such a type IV specific collagenase could play a role in tumor invasion and may be secreted by other cells such as endothelial cells, epithelial cells, and immune cells.