In 128 patients (20 f., 98 m., age between 26 and 88 years, mean age 59 years) with acute myocardial infarction, infarct size was calculated from CK and MBCK serum activity using the individual fractional decay rate kD. Only a moderate correlation could be found between infarct size calculated from CK-serum curves (CK-IG) and from MBCK (MBCK-IG) (r = 0.65). Only little improvement was achieved by excluding those patients who had been resuscitated (n = 13, r = 0.69). In 20% of the patients there was a good correlation (+/- 5 g eq) between CK-IG and MBCK-IG. In 14% MBCK-IG was larger and in 66% smaller than CK-IG. Calculation of MBCK-IG is based on a constant MBCK percentage of CK in myocardium, namely 14%, and assumes that the distribution volume and amount of enzyme released into the serum is the same for CK and MBCK. Thus the percentage of MBCK in serum compared to CK should also be 14%. We found an average value approaching this (13.6%), but with a wide range between 4.7 and 21.2%. It is this variation which is responsible for the poor correlation between both IGs.