This article reviews our current knowledge of the role of oxygen free radicals (OFR) in the process of arachidonic acid metabolism and platelet activation. During this activation several platelet enzymatic products are formed, which are measured by radioimmunoassays (RIA) and radioimmunometric assays (IRMA). Many studies have indicated that platelets are able to produce OFR, which are likely to play a significant role in the mechanisms of platelets activation and aggregation. These findings are important because they show that OFR may have a significant role in the mechanism of atherosclerosis and thrombosis. The role of several antioxidant factors in platelets' activation and aggregation is also presented in this review. It was found that antioxidant substances which act like OFR "scavengers" cause inhibition of arachidonic acid derivatives production and also inhibition of platelets activation. These studies suggest the possible therapeutic intervention with antioxidants acting as antiplatelet agents, to improve the pharmacological effects of various antiplatelet drugs. Finally we present our studies related to the arachidonic acid metabolites. The determination of arachidonic metabolic derivatives as thromboxane A(2), prostaglandin E(2), prostacyclin I(2) and isoprostane 8-iso-PG F(2alpha) by RIA and IRMA tests is important for the actual study of the above metabolic mechanisms because these tests are more accurate and less expensive, as compared to routine ELISA and other similar tests used for the same purpose.