Summary: Serum and plasma myoglobin and creatine kinase-MB catalytic activity were analysed in 157 patients admitted within 2 hours of the onset of ehest pain (58 were retrospectively recognized s acute myocardial infarction).Serum and plasma values were highly correlated for both myoglobin and creatine kinase-MB.Plasma myoglobin appared to be more sensitive than creatine kinase-MB for the early diagnosis of acute myocardial infarction; using a cut-off value of 100 μ §/1, 90% of acute myocardial infarction cases were correctly recognized by plasma myoglobin 6 hours after the onset of ehest pain, with a diagnostic specificity of 100% for non-acute myocardial infarction ehest pain subjects. Plasma creatine kinase-MB showed a diagnostic sensitivity of 62% and a diagnostic specificity of 95% in the same group of patients.We suggest the inclusion of the plasma myoglobin immunonephelometric assay together with plasma creatine kinase-MB activity analysis in protocols for the early diagnosis of acute myocardial infarction.