Using 4-nitorophenol (pNP)-glycopyranosides as pseudosubstrates, the amount of liberated pNP from the 4-Nitrophenyl α-D-galactopyranoside is significantly higher than that from the other pNP-glycopyranosides at pH 4.0 whereas no activity was detected at pH 7.0. The digestion assays using 4-nitrophenyl (pNP) glycopyranosides as artificial substrates shows that yieldin liberated galactose residues from 4-Nitrophenyl α-D-galactopyranoside mainly at pH 4.0, i.e. the pH level where yield threshold is decreased at most.