Because the concentration of serum myoglobin (Mb) increases within 2 to 4 hours after the first sign of acute myocardial infarction, it has been proposed as an early marker of the condition. Our aim was to evaluate a new assay that provides a rapid, quantitative determination of Mb (Baxter Stratus Myoglobin) based on the radial partition technique. We compared the results obtained by this technique with those from nephelometric and radioimmunoassay methods. A significant agreement was observed, the correlation coefficients (r) being 0.999 and 0.996, respectively. The method evaluated provided good reproducibility with CVs between 3.14% and 4.87%, and its linearity and analytical sensitivity were satisfactory. The clinical evaluation of this assay demonstrates that Mb increases in serum of patients with acute myocardial infarction before total creatine kinase and creatine kinase MB isoenzyme. Mb concentration shows an early peak and earlier return to normal values after the necrosis compared to enzymatic activities. Moreover the assay is rapid and fully automated. The method is therefore considered appropriate for contributing to the early diagnosis of AMI in clinical laboratories.