1. Human platelet-rich plasma incubated at 37°C generates an antiaggregatory prostaglandin which is spasmogenic on gastrointestinal smooth muscle.

2. Platelet-rich plasma from female donors generated more biological activity and was more sensitive to the anti-aggregatory activity of added prostaglandin I2 (PGI2) compared with that from age-matched male controls.

3. After thin-layer chromatography of extracted platelet-rich plasma, biological activity was detected in a zone which co-chromatographed with 6-oxoprostaglandin E1.

4. Neither extracted platelet-rich plasma nor authentic 6-oxoprostaglandin E1 were inactivated following incubation with purified 15-hydroxyprostaglandin dehydrogenase.

5. The relevance of these findings for regulating platelet reactivity is discussed.

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