SummaryAfter having reviewed the various biological tests utilized for the diagnosis of myocardial infarction and concluded to their lack of specificity, we report our experience with the determination in blood of phospho-creatin-kinase (P.C.K.) which is a specific muscular and myocardial enzymeWe have found an elevation of the blood level of P.C.K. only in diseases causing a necrosis of skeletal or heart muscles.By contrast, we have found a normal level of P.C.K. in a whole series of pathological conditions, including coronary sclerosis. The rise of the serum level of P.C.K. appears on the first day, as for SGOT, but does not persist more than 48 to 72 hours after myocardial infarction.Therefore we conclude that the determination of the serum level of P.C.K., in view of the specificity of P.C.K. for muscular tissue has a practical interest in the precocious detection of myocardial infarction. Nevertheless the determination of the sedimentation rate and the C.R.P. remains useful to follow the evolution of myocardial infarct.