Nine carboxylic acids were tested to evaluate their effects on recombinant fusion protein production and cell growth of Chinese Hamster Ovary (CHO) cells. Pentanoic acid was demonstrated to have the highest enhancement effect on the protein biosynthesis of CHO cells among the acids tested. Pentanoic acid also had less growth suppression effects compared with butyrate. The optimal induction time and concentration of pentanoic acid for a 120-h batch culture were 72 h and 1 mM, respectively. Apoptosis (programmed cell death) was observed in the serum-free batch culture of CHO cells using a cell death detection ELISA kit. The addition of butyrate accelerated the rate of apoptosis of CHO cells whereas the addition of pentanoate did not. These results confirmed that pentanoic acid was a better stimulant for protein biosynthesis in animal cell culture than butyrate.