Abstract: The invention relates to a Neutrokine-alpha protein in crystalline form, a method of preparing a Neutrokine-alpha protein in crystalline form, and methods of using a Neutrokine-alpha protein in crystalline form. In particular, the three-dimensional structure of a Neutrokine-alpha protein in crystalline form is used to design molecules that have biological activity. The methods are useful for designing compounds that bind to a Neutrokine-alpha protein, inhibit a Neutrokine-alpha protein, mimic a Neutrokine-alpha protein, and/or enhance the activity of a Neutrokine-alpha protein. The three-dimensional structure of a Neutrokine-alpha protein, as provided herein, is also used to determine the three-dimensional of other Neutrokine-alpha proteins and homologues thereof.