P-selectin is stored preformed in the alpha-granules of platelets. Previous studies show ß-thromboglobulin, also stored in alpha-granules can be readily released from platelets during the processing of whole blood. This artefactual release was rectified by using combination of various anti-platelet and anticoagulant compounds placed in the collecting tube. We investigated the levels of sP-selectin from 40 volunteers, comparing two anticoagulants, tri-sodium citrate, CTAD (a mixture of sodium citrate and citric acid, theophylline, adenosine and dipyridamole) plus iloprost and serum. Iloprost, a stable prostacyclin analogue, is a potent anti-platelet agent. We found significantly lower levels of sP-selectin (P<0.0001, paired t-test) measured from blood collected into CTAD and iloprost compared to levels measured from either citrated plasma or serum. We suggest that plasma levels obtained from the blood collected into a CTAD tube containing iloprost are likely to more accurately reflect the true levels of circulating sP-selectin than those obtained when test-tube activation of platelets is allowed to continue in vitro.