Glutathione reductase [NAD(P)H:GSSG oxidoreductase EC 1.6.4.2] from cyanobacterium Spirulina maxima was purified 1300-fold to homogeneity by a simple three-step procedure involving ammonium sulfate fractionation, ion exchange chromatography on DEAE-cellulose, and affinity chromatography on 2′,5′-ADP-Sepharose 4B. Optimum pH was 7.0 and enzymatic activity was notably increased when the phosphate ion concentration was increased. The enzyme gave an absorption spectrum that was typical for a flavoprotein in that it had three peaks with maximal absorbance at 271, 370, and 460 nm and a E 271 1% of 23.3. K m values were 120 ± 12 μM and 3.5 ± 0.9 μM for GSSG and NADPH, respectively. Mixed disulfide of CoA and GSH was also reduced by the enzyme under assay conditions, but the enzyme had a very low affinity ( K m 3.3 mM) for this substrate. The enzyme was specific for NADPH. The isoelectric point of the native enzyme at 4 °C was 4.35 and the amino acid composition was very similar to that previously reported from other sources. The molecular weight of a subunit under denaturing conditions was 47,000 ± 1200. Analyses of pure enzyme by a variety of techniques for molecular weight determination revealed that, at pH 7.0, the enzyme existed predominantly as a tetrameric species in equilibrium with a minor dimer fraction. Dissociation into dimers was achieved at alkaline pH (9.5) or in 6 m urea. However, the equilibrium at neutral pH was not altered by NADPH or by disulfide reducing reagents. The M r and s 20,w of the oligomeric enzyme were estimated to be 177,000 ± 14,000 and 8.49 ± 0.5; for the dimer, 99,800 ± 7000 and 5.96 ± 0.4, respectively. Low concentrations of urea increased the enzymatic activity, but this increase was not due to changes in the proportions of both forms.