There has long been discussion concerning the relationship between platelet activation and thromboxane (Tx) A₂ formation¹. Adapting the Born platelet aggregometry method for use in 96-well plate readers^2,3 has allowed us to demonstrate a linear relationship between platelet aggregration and thromboxane A₂ production when arachidonic acid is used as an agonist⁴. Here we have continued to study whether the same relationship exists when collagen is used as an agonist, taking into account that thromboxane (Tx) signalling is only necessary for aggregation when lower concentrations of collagen are used⁵. Human blood was collected by venepuncture into tri-sodium citrate (3.8% w/v final) and centrifuged to obtain platelet rich plasma (PRP). The PRP was then added to the wells of 96 well plates in the presence of increasing concentrations of aspirin (0.01-1000µM) or vehicle and incubated for 30min at 37°C before the addition of collagen (1 or 3μg/ml) or vehicle. Plates were then immediately placed in a 96-well plate reader and absorbance determined at 595nm every 15s for 16min between vigorous shaking. Changes in absorbance were converted to % aggregation by reference to the absorbances of PRP and platelet free plasma. At the end of the aggregatory responses plasma samples were removed and retained for analysis by radioimmunoassay of the content of TxB₂, as a measure of platelet TxA₂ formation. Platelet aggregation (72±1% of maximum and 84±1% of maximum in response to collagen 1μg/ml and 3μg/ml, respectively) and TxA₂ production were both inhibited in concentration-dependent manners by aspirin. The log M IC₅₀ values for aspirin as an inhibitor of collagen-induced platelet aggregation (-5.5±0.3 and -4.7±0.3, respectively) and TxA₂ production (-5.3±0.5 and -5.2±0.7, respectively) were not significantly different (n=8 for all). Furthermore, under all conditions there was a direct linear relationship between concentrations of TxA₂ and platelet aggregation. Here, therefore, we demonstrate a linear relationship between platelet aggregation and TxA₂ production in this model of aspirin and collagen activation of platelets. Such a relationship has important implications for our understanding of aspirin and NSAID therapy.