1. At present it is unclear whether platelet-activating-factor (PAF)-induced aggregation is mediated by thromboxane. To obtain further information about this event we have compared the affects of aspirin on platelet aggregation and secretion induced by PAF and collagen.

2. Collagen and PAF induced aggregation and secretion in human platelets in a dose-related manner.

3. Aspirin inhibited the magnitude of both platelet aggregation and secretion induced by PAF and collagen, but the degree of inhibition was much greater for collagen.

4. Aspirin strongly inhibited the aggregation rate of collagen-induced platelet aggregation, but had no measurable effect on the rate of PAF-induced aggregation.

5. Inconsistencies reported in previous studies of the effect of aspirin on PAF-induced platelet aggregation may be explained, in part, by the doses of PAF used and the method of inactivating cyclo-oxygenase (in vitro compared with in vivo).

6. Our results suggest that the initial events of PAF-induced aggregation are independent of thromboxane A2 formation and that thromboxane A2 plays only a minor role in the later phase of PAF-induced aggregation.

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