In samples of human aortic intima fibrin/fibrinogen degradation products (FDP) were assayed by isoelectric focussing/immunoelectrophoresis as a possible measure of endogenous fibrinolysis, and plasminogen concentration was assayed by rocket immunoelectrophoresis as a possible marker for fibrinolytic potential. No consistent differences were found between normal intima and different types of atherosclerotic lesion, but there was marked variation between patients, and multiple samples from the same aorta showed similar levels. There was no significant correlation with age, sex, or time after death. Low concentrations of FDP and failure to recover measureable amounts of plasminogen from intima were highly associated with death in patients who had suffered a recent myocardial infarction. In aortas from which 3 or more samples of intima and lesions were obtained (n = 16), no FDP were found in 3 (total of 12 samples); all of these were from patients who died following myocardial infarction. Low levels were present in the 4th patient with myocardial infarction. No plasminogen was found in 10 of 11 aortas from patients dying after myocardial infarction (total of 46 samples with no plasminogen), but it was present in 10 of 17 aortas from patients dying of other causes (X2 = 7.6, P less than 0.01). Where both were assayed, FDP were not found in any samples which did not contain plasminogen. Low levels of FDP and absence of plasminogen were associated with increased involvement with atherosclerosis. There was no relation between intimal and serum plasminogen levels, and prothrombin and low density lipoprotein were present in all samples from which no plasminogen was recovered. The results indicate that in some patients, particularly those dying after myocardial infarction, there is decreased fibrinolysis and fibrinolytic potential in the arterial intima, and this may result in increased intimal accumulation of fibrin.