Previous investigations have shown that members of the genus Vibrio possess a novel enzyme activity decarboxylating L-2,4-diaminobutyric acid (DABA) to 1,3-diaminopropane (DAP). In this paper we describe the purification, by about 3600-fold, of the enzyme from V. alginolyticus. The purified enzyme was apparently homogeneous, and had a specific activity of 4200 nmol DAP min-1 (mg protein)-1. The enzyme protein has an Mr of 450,000 +/- 20,000 and is apparently comprised of four identical subunits (Mr 109,000 +/- 1,000). Neither 2,3-diaminopropionic acid, ornithine, lysine nor arginine served as substrates. Some properties of the enzyme were determined. Cultivation of this bacterium in the presence of added DABA brought about increased production of norspermidine (NSPD), characteristically present in this species as well as DAP, suggesting that the enzyme may be functionally implicated in the formation of DAP, a biosynthetic precursor of NSPD.