Prostacyclin (PGX) strikingly increases cyclic AMP concentrations in human platelets. Prostacyclin is approximately 10 times more active than PGD2, 30 times more active than PGE1 and more than 1000 times more active than its stable end product, 6-oxo-PGF1alpha. These results correlate well with the anti-aggregating activity of prostacyclin, compared with PGE1 and PGD2.