Protein aggregation is problematic in various fields, where aggregation can frequently occur during routine experiments. This study showed that tetraethylene glycol (TEG) and tetraethylene glycol dimethyl ether (TEGDE) act as aggregation suppressors that have different unique properties from typical additives to prevent protein aggregation, such as arginine (Arg) and NaCl. Thermal aggregation of -chymotrypsin was well suppressed with the addition of both TEG and TEGDE. Interestingly, the suppressive effects of Arg and NaCl on thermal aggregation were almost unchanged when temperature was shifted from 65 degrees C to 85 degrees C, whereas both TEG and TEGDE significantly decreased the aggregation rate with increasing temperature. Note that the effects of TEG and TEGDE were higher than Arg above 75 degrees C. This temperature-dependent behavior of TEG and TEGDE provides a novel design guideline to develop aggregation suppressors for use at high temperature, i.e., the importance of the ethylene oxide group. (c) 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:1325-1330, 2013