Arginine decarboxylase (EC 4.1.1.19) was purified to homogeniety from the cytosol of soybean (Glycine max) axes. The enzyme was amplified by acid stress and purified by ammonium sulfate precipitation, acetone precipitation, gel filtration and arginine-Sepharose 4B affinity chromatography. The enzyme was free from arginase and urease. The enzyme was quite labile but dramatically stabilized by the addition of 0.1% Tween 80. The molecular weight was determined to be 269,000 dalton by sephacryl S-300, while SDS-PAGE gave two bands at a molecular weight of 72,400 and 63,000 dalton, indicating that the enzyme was a heterotetramer (α₂β₂). The enzyme was highly specific for L-arginine and its K_m value for L-arginine was 60.13 μM. Agmatine, the product of the enzyme, was a competitive inhibitor and K_i value for it was 20 μM. Drasitic inhibition by agmatine strongly suggests that it may play a vital role in the regulation of polyamine biosynthesis through the inhibition of arginine decarboxylase as will as S-adenosylmethionine decarboxylase in soybeans.