The method for the determination of lactate dehydrogenase (LD) isoenzymes and estimation of the ratio of the cardiac specific LD isoenzymes one and two has been improved and applied to the analysis of serum samples from patients for the detection and differential diagnosis of myocardial infarction (MI). General medical patients have been studied as well as patients undergoing heart operations. The improved reliability and diagnostic discrimination of the LD isoenzyme test has provided valuable diagnostic information in the detection of MI and other complications in these patients.