To investigate the tolerance factor of Saccharomyces cerevisiae KNU5377 against various types of environmental stress during fermentation, we identified the protein that is upregulated at high temperatures. The highly upregulated protein was high-score-matched as a cytoplasmic peptidyl-prolyl cis-trans isomerase, cyclophilin (Cpr1p), by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF). We constructed a CPR1-deleted KNU5377 strain (KNU5377Y cpr1Delta) to determine the roles of the protein under fermentative or stress condition. The growth of the S. cerevisiae KNU5377Y cpr1Delta strain was completely inhibited under the following conditions: heat (40 degrees C), hydrogen peroxide (20-30 mM), menadione (0.3 mM), ethanol (16%), sulfuric acid (5 mm), and lactic acid (0.4-0.8%). However, the wild-type and cpr1Delta mutant of S. cerevisiae BY4741 as a positive control did not show differences in sensitivity to stress. It is interesting to note that the wild-type KNU5377Y and KNU5377Y cpr1Delta mutant showed high sensitivity against various stresses, particularly, acid stress such as in the presence of sulfuric and lactic acid. Although the alcohol fermentation rate of the KNU5377Y cpr1Delta mutant markedly decreased with an increase in temperature up to 40 degrees C, we observed no decrease in that of the wild-type strain under the same conditions. These results suggest that CPR1 contributes to the stress tolerance of KNU5377 against various types of environmental stress caused during fermentation, thus leading to the physiological role of maintaining an alcohol fermentation yield, even at high temperatures such as 40 degrees C.