Hydroxypyruvate has been shown to be an alternative substrate for lactate dehydrogenase. The clinical value of this "glycerate dehydrogenase" has been investigated on 60 patients admitted to the hospital with a provisional diagnosis of myocardial infarction. For comparison, lactate dehydrogenase, α-hydroxybutyric dehydrogenase, creatine kinase, aspartate transaminase, and alanine transaminase were also assayed at 37°C. Normal ranges and methods of assay are given for each enzyme. Glycerate dehydrogenase has greater activity than any other enzyme and parallels lactate dehydrogenase and α-hydroxybutyric dehydrogenase in its time of peak activity and duration of increased activity. Clinical and biochemical evidence is provided to show that α-hydroxybutyric dehydrogenase is not specific for the cardiac isoenzymes. Of the enzymes studied, creatine kinase activity increases most quickly after myocardial infarction, and an exclusive role for its use in coronary care units is also suggested.