The amino-terminal fragment (ATF, Ser1-Glu143) of urokinase-type plasminogen activator (uPA) is responsible for some important functions of uPA, such as receptor binding and chemotactic activity. To dissect the function and structure-activity relationship of ATF, recombinant human ATF was expressed in Pichia pastoris system at a yield of about 30 mg/L. The recombinant ATF was captured by a cation exchange column, further purified up to 99% purity by a gel filtration column, and characterized in terms of its receptor binding capability. The purified ATF was then crystallized by the method of sitting-drop vapor diffusion with magnesium sulfate as the precipitating agent at 298 K. The crystals belong to space group P1 with unit cell dimensions of a=47.5A, b=64.7A, c=65.4A, alpha=71.6 degrees , beta=92.1 degrees , gamma=84.0 degrees .