1. The enzymic oxidation of glyoxylate to oxalate in the soluble (100 000 g supernatant) fraction of liver and heart tissue from a patient with primary hyperoxaluria and from a non-hyperoxaluric subject have been studied.
2. An oxidized nicotinamide—adenine dinucleotide (NAD+)-dependent and a non-NAD+-dependent oxidation of glyoxylate to oxalate were observed in the liver tissue from both sources.
3. Evidence is presented that lactate dehydrogenase has a major role in catalysing the reaction in both of the tissues studied. The non-NAD+-dependent oxidations which are catalysed by xanthine oxidase and glycollate oxidase in the liver are relatively unimportant, and they were not detected in the heart.
4. An enzyme that catalyses the oxidation of glycollate was also demonstrated in liver tissue. This had a different electrophoretic mobility from the lactate dehydrogenase isoenzymes.
5. These findings are discussed with particular reference to human primary hyperoxaluria in which excessive oxalate synthesis occurs.