Oxamate, a potent inhibitor of lactate dehydrogenase, is shown also to inhibit aspartate aminotransferase activity, both in human serum and in purified isoenzymes of human origin. The inhibition was competitive with respect to 2-oxoglutarate for both isoenzymes. The apparent Ki was 29 mmol/L for the cytoplasmic enzyme and 17 mmol/L for the mitochondrial enzyme. Noncompetitive inhibition was found between oxamate and aspartate. At saturating concentrations of substrate (2-oxoglutarate greater than or equal to 15 mmol/L, L-aspartate greater than or equal 150 mmol/L) oxamate inhibited the mitochondrial enzyme but had less effect on the cytoplasmic isoenzyme. Oxamate at 40 mmol/L inhibited the enzyme in serum by 11 and 9% in assays containing 2-oxoglutarate at 6.7 and 15 mmol/L, respectively. This concentration of oxamate inhibited enzyme activity in serum by 5% more than did the same concentration of Cl- (itself an inhibitor). Oxamate (less than or equal to 30 mmol/L) had no measurable effect on the stability or activity of porcine malate dehydrogenase. Until the effects of its inhibitory properties are considered, addition of oxamate to suppress lactate dehydrogenase-mediated side reactions in the assay of aspartate aminotransferase cannot be recommended.