Proteinase inhibitors comprising a Kunitz domain are disclosed. The Kunitz domain comprises a sequence of amino acid residues as shown in SEQ ID NO:3, wherein the sequence is at least 80% identical to residues 6 through 56 of SEQ ID NO:2. Also disclosed are methods for making the proteinase inhibitors, and expression vectors and cultured cells that are useful within the methods. The proteinase inhibitors may be used as components of cell culture media, in protein purification, and in certain therapeutic and diagnostic applications.