L-Argininosuccinate arginine-lyase (EC 4.3.2.1) was extracted from the cotyledons of germinating pea seeds. The enzyme was purified approximately 20 times by isoelectric precipitation, followed by treatment with calcium phosphate gel and chromatography on carboxymethyl cellulose. The partially purified enzyme was shown to catalyze both the condensation of arginine and fumarate to give argininosuccinate and the cleavage of argininosuccinate to give arginine and fumarate. The enzyme displayed maximal activity at pH 7.9 and was relatively stable after storage at −20 °C for 12 months or after dialysis overnight at 2 °C. The Michaelis constants for argininosuccinate and arginine were found to be 2 × 10−4 M and 6.7 × 10−3 M respectively. Enzyme activity was partially inhibited by 10−4 M p-chloromercuribenzoate. It is concluded that this enzyme has importance in arginine biosynthesis in germinating pea cotyledons.