• Serum aminopherase levels were determined in 77 patients with the unequivocal diagnosis of myocardial infarction and were compared with the levels found in 73 patients with other diseases. A strong association was found between myocardial infarction and high serum aminopherase levels. The occasional finding of a low aminopherase level in the undeniable presence of myocardial infarction (false negative) was explained by the occasionally transient nature of the elevation; the danger of confusion can be reduced by sufficiently prompt and frequent determinations. The finding of a high aminopherase level in the absence of myocardial infarction (false positive) was also observed, especially in disease of the pancreas and liver.