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Preparations of reduced NAD are known to be prone to contain a substance which inhibits the catalytic activity of LDH. (1-Lactate: NAD oxidoreductase,EC 1.1.1.27). P-f HIS PUBLICA’I’ION presents data concerning the relative quality of commercially available reduced NAP preparatioums for LDH assays. 1)ata on the influence of the inhibitor on otimer dehydrogenases are presented, as well as data concerning time inhibitor in coupled enzyme tests with dehydrogenases as indicator enzymes. Freshly prepareci solutions of various commercially available re-duced NAT) preparations were foummd to l)e essentially free of inhibitors acting on dehydrogenases. Tt is show-mmthat time dehydrogenases GLT)T1 (14-glutamate:NAT) oxidoreductase, deaminating, EC 1.4.1.2), ‘‘a-HBDH’ ’ (‘‘-hydroxybutyrate (lelmydrogenase’’), LI)H (L-lactate: NA1) oxidoreductase, EC 1.1.1.27), MDH (L-malate:NAD oxidore