Serum guanase, aspartate aminotransferase, alanine aminotransferase, creatine phosphokinase and hydroxybutyrate dehydrogenase activities were measured in 290 blood samples from 96 consecutive patients admitted to a Coronary Care Unit. Elevated serum guanase activities (greater than 2 U/l) were found in 19 patients (20%). The magnitude and frequency of these elevations did not negate the value of guanase as a "liver function test", since all cases with raised guanase also had abnormal serum alanine aminotransferase activities. This fact, together with other information in the literature, indicated that elevated serum guanase activity following myocardial infarction was consequent upon some degree of sub-clinical hepatic necrosis. Caution must be exercised when serum asparate aminotransferase is used as an index of heart muscle necrosis unless guanase or some other "liver specific" enzyme is known to be normal, or unless creatine phosphokinase or hydroxybutyrate dehydrogenase activities are elevated.