The diagnostic value of serum myoglobin as compared to MB iso-enzyme of creatine phosphokinase and aspartate aminotransferase was investigated in 25 patients admitted on suspicion of acute myocardial infarction with a duration of symptoms less than 6 hours. In group 1 (acute myocardial infarction group), the first blood sample, obtained at a mean time of 3.27 hours after onset of infarction, invariably showed increased myoglobin (mean 2.6-fold normal) whereas MB iso-enzyme of creatine phosphokinase and aspartate aminotransferase were often normal. Peak myoglobin values occurred earlier than peak serum MB iso-enzyme of creatine phosphokinase values. The highest peak values of serum myoglobin were found in patients with extensive myocardial infarction. In group 2 (non-acute myocardial infarction or control group) serial determinations of serum myoglobin, serum MB iso-enzyme of creatine phosphokinase and aspartate aminotransferase were within normal limits. Hence the importance lies with the early detection of serum myoglobin in acute myocardial infarction.