Biliverdin reductase was purified from cow spleen. The specific activity of the final enzyme preparation was 24.01 u/mg, representing 686-fold purification as measured with NADPH. The yield was 3 grams of enzyme per 100 grams of cow spleen. The purified enzyme was a monomeric protein with an apparent molecular weight of about 34,000 and an isoelectric point of about 6.2. The biliverdin reductase was specific for biliverdin and reduced IX alpha faster than the biliverdin isomers IX beta, IXr, or IX delta. The purified enzyme could utilize both NADH and NADPH, but the kinetic properties of the NADH-dependent and the NADPH-dependent enzyme activities were different: the time course of the NADPH-dependent reaction displayed a sigmoidal curve, whereas that of the NADH-dependent reaction did not. Km for biliverdin IX alpha was 4 x 10(-4) mM in the NADPH system, while it was 1.5 x 10(-3) mM in the NADH system. Both enzyme activities were inhibited by excess biliverdin, but the inhibition of the NADPH-dependent enzyme activity was more pronounced. The pH optimum was 7.0 with NADH, and 6.8 with NADPH.