Lactate dehydrogenase (LD: EC 188.8.131.52) is the most important clinically of several dehydrogenases occurring in human serum. Lactate dehydrogenase is cytoplasmic in its cellular location and in any one tissue is composed of one or two of five possible isoenzymes. While many of its clinical applications involve quantification of one or more specific serum isoenzymes, an estimate of total LD is required usually. Lactate dehydrogenase catalyzes the reversible reaction: L-lactate + NAD+ in equilibrium pyruvate + NADH. The bidirectional reaction is monitored spectrophotometrically by measuring either the increase in NADH at 340 nm produced in the lactate-to-pyruvate reaction (L----P) or by the decrease in NADH at 340 nm produced in the pyruvate-to-lactate (P----L) reaction. Kinetic assay systems for the measurement of the reaction system in both directions are comprehensively reviewed as well as the standardization efforts proposed to date.