Protectants (stabilizers) are added to protect proteins during lyophilization and/or long-term storage. Proteins are subjected to various stresses generated by lyophilization which may cause loss of activity, aggregation, or denaturation. Protectants can alleviate the stresses by several mechanisms including formation of an amorphous glassy state, replacing water, hydrogen bonding to proteins, physical dilution and separation of protein molecules, etc. Common protectants/stabilizers include sugars, polyols, polymers, surfactants, as well as some proteins and amino acids. We use trehalose and mannitol (normally 8% w/v) as protectants for lyophilization. Trehalose is a non-reducing disaccharide, well-known for its ability to stabilize biomolecules and microorganisms during prolonged period of desiccation. Mannitol is also commonly used as a stabilizer as well as a bulking agent in the process of lyophilization. It was reported to reduce aggregation for some proteins during lyophilization.