We claim the proteinase inhibitors comprising a Kunitz domain, wherein the Kunitz domain comprises amino acid residues 57 107 of SEQ ID NO: 2. We also claim a fusion protein comprising said Kunitz domain. In addition, we claim a method of inhibiting protease degradation or protease activity in a composition which contains plasma proteins and proteases. The proteinase inhibitors is used as components of cell culture media in cell culture process, and as inhibitors of protease-mediated degradation of plasma proteins in protein purification.