The usefulness of specific enzyme reactions to clinical medicine is becoming increasingly more apparent. Patients suffering destruction of various tissue cells have increased enzymatic activity in their serum. The enzymes normally present within the cells are liberated into the blood stream as cell destruction occurs. The determination of serum lactic dehydrogenase (LDH) as an aid in the diagnosis of acute myocardial infarction has two advantages: first the increased elevation is present for as long as six to eight days after attack; second, the technical procedure is far less complicated than other enzyme tests. LDH determination, not ordinarily necessary in the diagnosis of acute myocardial infarction, is particularly helpful when the electrocardiograph pattern masks the recent changes of an acute infarction.