Antarctic sea ice diatoms produce ice-binding proteins (IBPs) that are strong inhibitors of the recrystallization of ice. Their function may be to reduce cell damage in the frozen state. We show here that an IBP from the diatom Navicula glaciei Vanheurck also has the ability to reduce freeze-thaw damage to red blood cells and that the effect may be due to its ability to inhibit recrystallization of ice.