Using ammonium sulfate precipitation, gel filtration, and affinity chromatography, inosine monophosphate (IMP) oxidoreductase (EC 1.2.1.14) was isolated from the soluble proteins of the plant cell fraction of nitrogen-fixing nodules of cowpea (Vigna unguiculata L. Walp). The enzyme, purified more than 140-fold with a yield of 11%, was stabilized with glycerol and required a sulfydryl-reducing agent for maximum activity. Gel filtration indicated a molecular weight of 200,000, and sodium dodecyl sulfate-gel electrophoresis a single subunit of 50,000 Da. The final specific activity ranged from 1.1 to 1.5 mumol min-1 mg protein-1. The enzyme had an alkaline pH optimum and showed a high affinity for IMP (Km = 9.1 X 10(-6) M at pH 8.8 and NAD levels above 0.25 mM) and NAD (Km = 18-35 X 10(-6) M at pH 8.8). NAD was the preferred coenzyme, with NADP reduction less than 10% of that with NAD, while molecular oxygen did not serve as an electron acceptor. Intermediates of ureide metabolism (allantoin, allantoic acid, uric acid, inosine, xanthosine, and XMP) did not affect the enzyme, while AMP, GMP, and NADH were inhibitors. GMP inhibition was competitive with a Ki = 60 X 10(-6) M. The purified enzyme was activated by K+ (Km = 1.6 X 10(-3) M) but not by NH+4. The K+ activation was competitively inhibited by Mg2+. The significance of the properties of IMP oxidoreductase for regulation of ureide biosynthesis in legume root nodules is discussed.