Abstract: Frog albumin (XpA-serum) was purified from serum of Xenopus laevis by a combination of gel filtration and ion exchange chromatography steps. Like Bombina maxima albumin, XpA-serum exhibited trypsin inhibitory activity, which was lower than that of B. maxima albumin. XpA-serum at the concentration of 180 nM inhibited 84% activity of trypsin (30 nM). The equilibrium dissociation constants (KD) is 1.44×10-6 M as determined by Surface Plasmon Resonance. Western blot analysis revealed that XpA-serum was also distributed in the skin. It is deduced that serum albumin of amphibian possessing trypsin inhibitory activity can function directly or indirectly as a defensive substance against predators.