As whole blood passes through a column of glass beads, platelets are activated and all 3 basic platelet properties (adhesion, release reaction and aggregation) displayed, an aspect similar to platelets being involved in a hemostatic process. For this reason, we chose the platelet retention test as the tool to study the effect of synthetic prostacyclin (PGI2) on platelet function and behavior. PGI2inhibited platelet retention with its major impact on platelet aggregation. The extent of inhibition was not proportional to the concentration of PGI2. It appears that 2 levels of platelet aggregation take place inside the column. One, triggered by a weak stimulus, is readily inhibitable by low concentrations of PGI2. The other, brought about by a strong stimulus, requires higher concentrations of PGI2to achieve complete inhibition. Even with as much as 100 ng PGI2/ml blood, a 15% retention remained which probably represented platelet adhesion to glass beads. Scanning electron microscopic examination of glass beads showed massive platelet aggregation and adhesion in control blood samples. No platelet aggregate was observed on beads filtered with blood containing 20 ng/ml or more PGI2. Individual platelets were found on glass beads filtered with samples containing any concentration of PGI2; these platelets exhibited extensive morphologic changes.