Some properties of poly(ADP-ribose) synthetase, which catalyzes the transfer of ADP-ribose moiety of NAD to accepter protein, were characterized in the sprout of Glycin Max Linne (soybean). The enyme showing the maximal activity at pH 8.0 was largely distributed in the root area and heat-unstable by complete inactivation at 55℃ for 10 minutes. Poly (ADP-ribose) synthetase activity increased gradually by the incubation and also increased about 50% when the enzyme was preincubated. at 37℃ for 3 hours with or without NAD, indicating that it is activated by some factors other than NAD during the incubation. The enzyme activity was proportionate to the enzyme amount within the small range but dramatically reduced in the high concentrations of enzyme. suggesting that the enzyme preparations contained a factor affecting the synthesis or degradation of poly(ADP-ribosyl)-protein.