In this study, the presence of paraoxonase activity in pericardial fluid was demonstrated. A comparison of some properties, such as optimum pH, stability versus pH, heat inactivation, effect of inhibitors, isoelectric point and kinetic parameters (Km and Vmax), between plasma and pericardial fluid paraoxonase was made. The properties studied were practically identical. The enzyme activity in pericardial fluid was tested as a marker in the postmortem diagnosis of myocardial infarction. The paraoxonase activity in the myocardial infarction group (47 cases) was lower than in the control group (40 cases), but the difference was not significant.