Purified fibrinogen strongly acts as an antioxidant by inhibiting the chemiluminescent emission developed in vitro, in a cell-free system composed of luminol and hydrogen peroxide. The antioxidative action of fibrinogen depends directly on its concentration, even in the presence of human serum. On the other hand, the platelet aggregation is an important source of oxygen free radicals. These radicals can also be measured by chemiluminescence, when the platelet aggregation is triggered by arachidonic acid. In a platelet suspension, fibrinogen inhibits both aggregation and the associated chemiluminescent emission. A possible correlation between the plasma level of lipid peroxides and fibrinogen was studied in different groups of patients, mostly with cardiovascular diseases and neoplasms. A weak correlation was found only in cardiovascular diseases, in which the tendency of fibrinogen increase could also be interpreted as an antioxidative action of peroxidation restriction, especially in ischemic conditions. In neoplasms, this correlation could not be found, in spite of the high level of fibrinogen associated with a decrease of peroxide formation, a characteristic feature of tumoral growth due to the change of fatty acids nature in the cellular membranes.