<P>Aggregation activity of platelets in umbilical blood is lower than that in adult blood, but the reason for this is not well understood. It has recently been clarified that calcium plays a role as a second messenger of platelet aggregation, and that glycoproteins of platelet surface membrane such as glycoprotein I b and IIb/IIIa are receptors for agonists inducing aggregation. We examined the concentrations of intracellular calcium and the membrane glycoproteins of platelets in umbilical and adult blood. The increase of intracellular calcium in umbilical platelets was lower than that in adult platelets when the aggregation was induced by ADP, collagen, thrombin and epinephrine. Only calcium ionophore A23187 induced aggregation of both umbilical and adult platelets. On the other hand, there were no qualitative differences between glycoproteins I b and IIb/IIIa of these two groups. Therefore, the low aggregation activity of umbilical platelets seems to be due to low responsiveness of the intracellular calcium system, not to the disorder of functional surface membrane glycoprotein.</P></p
