In this study, we found that the denaturation of proteins during freezing is closely related to surface-induced denaturation. Several proteins with varying sensitivities to freezing were tested, and the results were compared with susceptibilities to surface denaturation in unfrozen aqueous solution. Also, the influence of the surfactant Tween 80 on the denaturation of each of the proteins was examined during freeze-thawing, as were the effects of Tween 80 and several other surfactants on the stability of lactate dehydrogenase. Proteins formed insoluble precipitates when they were subjected to a quench cooling by dipping in liquid nitrogen, although freezing followed by supercooling caused less precipitation. A strong correlation (r = 0.99) was observed between the tendency of a protein to freeze denature and its tendency to surface denature. Also, the addition of small amounts of surface-active agents protected proteins from both freeze- and surface-induced denaturation. Freeze-induced denaturation of IL-1ra at the ice-water interface during freeze-drying was effectively prevented by adding a small amount of Tween 80. These results suggest that the denaturation of proteins during freeze-thawing can be ascribed primarily to the increase in the area of the ice-water interface during freezing.