Platelet flow cytometry is an emerging tool in diagnostic and therapeutic hematology. It is eminently suited to assess the expression of platelet surface receptors both qualitatively as well as quantitatively. It can thus serve as a useful parameter for in vivo expression of platelet activation and thus fore-warn the risk of thromboembolism which is inherent in patients with untreated type 2 diabetes mellitus. This technique can also be used to study and compare the effect of various antiplatelet drugs on the level of platelet activation and to establish any dose-effect relationship of these drugs. Topographical localization of platelet granules and study of platelet-platelet and platelet-leukocyte interaction is also possible by this procedure. All these parameters serve as pointers towards the presence of activated platelets in the circulation with its thromboembolic consequences. This technique can also be put to therapeutics by assessing the effect of varying doses of clopidogrel on suppressing platelet activation. It can thus serve as a reliable and easy technique to determine the dose of various antiplatelet drugs and offer a custom-made protective antithrombotic therapy. This study was undertaken with the primary object of determining platelet reactivity in patients with untreated type 2 diabetes mellitus and to study the effect of clopidogrel administration on the state of in vivo platelet XIactivation in these patients. It was extended to include the effect of clopidogrel on reactive platelet CD markers after in vitro pre-treatment with platelet agonists, more specifically ADP. This agonist was selected because of its easy handling and predicable platelet stimulating effect. It was postulated that platelets in patients with type 2 diabetes mellitus circulate in the blood in hyperactive state and predispose to thrombotic complications so frequently encountered in these patients. This study confirms that platelets in these patients are at a higher state of reactivity and that clopidogrel can effectively suppress the enhanced platelet response. Four CD markers were chosen for this study; of these four markers CD63 and CD62p have a linear relationship with heightened platelet reactivity. The other two markers i.e. CD41 and CD61 are unaffected in these patients and do not seem to be causally related to increased propensity to develop athero-thrombotic complications in patients with type 2 diabetes mellitus. Sequential study of the reactive CD markers i.e. CD63 and CD62p shows that CD63 hyperactivity as indicated by flourescine positivity is a constant and persistent change while that of CD62p is an unstable response which falls off with the passage of time. It is therefore concluded that CD63 platelet markers should be evaluated to assess the state of platelet reactivity in patients with untreated type 2 diabetes mellitus.