Platelet aggregation is routinely measured by light transmission techniques¹ and this has been developed for use in 96-well plates². This method allows testing of many aggregatory responses over an identical short time period. When two or more platelet agonists are added together in low concentrations to platelet-rich plasma the individual effects of each other are enhanced³. We have evaluated the effects of the combined addition of collagen and adrenaline on aggregation of platelet-rich plasma using 96-well plate method. 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 or absence of collagen, adrenaline or combinations of collagen and adrenaline. Plates were then immediately placed in a 96-well plate reader and absorbance determined at 595nm every 15s for 16min with vigorous shaking. Changes in absorbance were converted to % aggregation by reference to the absorbance of PRP and platelet-poor plasma. Aggregatory responses to combined agonists were enhanced compared to collagen or adrenaline alone. For example at 4min, the aggregation induced by combination of 1µg/ml collagen and 10^-7 M adrenaline was 56±16%, compared to 1µg/ml collagen alone, 27±12%, or 10^-7 M adrenaline alone, 22±5% (n=4 for each). At 8min, combination of 10^-7 M adrenaline with 0.1, 0.3 or 1µg/ml collagen increased aggregation to 61±12%, 64±14% and 79±9% respectively compared to collagen alone (0.1µg/ml, 34±10%; 0.3µg/ml, 42±14%; 1µg/ml, 57±7%) or 10^-7 M adrenaline, 46±9% (n=4 for each). Using 96-well plates we demonstrated the additive effects of combinations of low concentrations of collagen and adrenaline on platelet aggregation in PRP. This approach could be useful since low concentrations of several agonists may better mimic the conditions under which thrombosis occur in vivo than single agonists.