The serum myoglobin (MG) was assayed by the radio-immunological method in 30 patients, all victims of a recent myocardial infarction (MI) and in 30 tests subjects suffering (21 cases) or not (9 cases) from heart diseases, but none from myocardial infarction (MI). The blood samples have been collected on hospital admission of the patient, then every four hours during the first 48 hours and finally, every 12 hours from the 48th to 72nd hour. The normal value is less than 85 micrograms/l. The creatine-kinase (CK), the aspartate aminotransferase (ASAT), the alanine aminotransferase (ALAT) and the lactate dehydrogenase (LDH) were also assayed each time. In MI, there is a significant increase in the serum MG level (731 +/- 323 micrograms/l against 174 +/- 198 micrograms/l in the test subjects; p less than 0.001). The sensitivity of this assay reaches 97%, its specificity 80%, its positive predictive value 83% and its negative predictive value 96%. Starting from the beginning of the characteristic pain of infarction, the MG level exceeds the normal values after 3.3 +/- 1.6 hours, reaches its maximum after 9.3 +/- 3.7 hours and comes back to normal after 38 +/- 8.1 hours. On the other hand, the MG level does not enable any conclusion regarding either the transmural/not transmural nature, or the site, or the acuteness of the MI.(ABSTRACT TRUNCATED AT 250 WORDS)