Addition of several arginine-specific serine proteases to culture medium conditioned by fibroblasts results in the proteases being taken into sodium dodecyl sulfate-stable complexes with a secreted factor termed protease nexin (PN) (Baker, J. B., Low, D. A., Simmer, R. L., and Cunningham, D. D. (1980) Cell 21, 37-45). PN not only inhibits these degradative enzymes but also mediates their binding, internalization, and degradation by the cells (Low, D. A., Baker, J. B., Koonce, W. C., and Cunningham, D. D. (1981) Proc. Natl. Acad. Sci. U. S. A. 78, 2340-2344). Here we describe a simple procedure for purifying milligram quantities of PN from serum-free medium conditioned by human foreskin cells. Accumulation of PN in the medium is increased by using high density microcarrier cultures supplemented with epidermal growth factor and bovine serum albumin. Application of ultrafiltration-concentrated medium to a heparin-Sepharose column followed by extensive washing of the column with buffer containing 0.2 M NaCl and elution with buffer containing 1.0 M NaCl results in the recovery of 60-90% of the input PN in a form that is 90-97% pure. This preparation can be further purified by hydrophobic chromatography on octyl-agarose. Purified PN has a molecular mass of approximately 51 kilodaltons. On nonequilibrium pH gradient electrophoresis it migrates as five bands with isoelectric points between 7.5 and 7.8. Purified PN exhibits all the properties attributed to PN in culture medium. These include: 1) formation of sodium dodecyl sulfate-stable complexes with thrombin, urokinase, and plasmin; 2) inhibition of protease activity; 3) heparin-enhanced inhibition of thrombin; and 4) cellular binding of protease-PN complexes in a heparin-sensitive reaction. When thrombin-PN complexes are dissociated with 1 M hydroxylamine a smaller form of PN (approximately 46 kilodaltons) is detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, indicating that the complexed PN is proteolytically modified.