Dihydropteridine reductase [EC 184.108.40.206] was purified from bovine liver in 50% yield and crystallized. The physicochemical properties of the purified enzyme were quite similar to those of sheep liver dihydropteridine reductase. During the course of purification, however, the enzyme was found to be separated into 2 major peaks together with minor peaks by column chromatography on CM-Sephadex, and one of the major peaks was identified as a binary complex of the enzyme with NADH. The reductase-NADH complex was also prepared in vitro and crystallized. Upon addition of quinonoid-dihydropterin to the complex, NADH was oxidized and released from the enzyme. The amount of bound NADH was calculated to be 2 moles per mole of the reductase. The occurrence of the reductase-NADH was calculated to be 2 moles per mole of the reductase. The occurrence of the reductase-NADH complex in bovine liver extract as a predominant form was in accord with the pyridine nucleotide specificity for NADH as a coenzyme. The results further support the view that NADH is the natural coenzyme of this reductase.