Protease nexin I is a 44-kD thrombin and urokinase inhibitor released by human foreskin fibroblasts. PN-I shares several features with antithrombin III , an abundant plasma thrombin inhibitor. Both PN-I and AT-III have high affinities for heparin, and heparin accelerates their rate of thrombin inhibition. In addition, the published sequence of 28 amino acids at the N-terminus of PN-I is identical to the N-terminal amino acid sequence of a glial-derived neurite promoting factor. Two forms, designated alpha and beta, were identified. These differed only by the insertion of 3 nucleotides into the coding sequence, and apparently arose by utilization of an alternative splice acceptor site in the PN1 gene. In the resulting proteins, alpha-PN-I contains an arginine residue at position 310, whereas beta-PN-I contains threonine-lysine residues.