Although platelets stored by cryopreservation are effective in hemostasis, they acquire a number of functional defects during storage and preparation for transfusion. In addition to known acquired defects such as defective aggregation, decreased resistance to hypotonic shock, and disc-spherocyte transformation, we have shown that cryopreserved platelets have decreased capacity to adhere to subendothelium, compared to liquid-stored platelets. To investigate this decrease in adhesive capacity of cryopreserved platelets, we measured the major adhesive membrane glycoprotein, GPIb, and the principal aggregatory protein, GPIIb/II- Ia, using flow cytometry in fresh platelets or in platelets cryopreserved in 5% DMSO. We also analyzed aggregation of cryopreserved platelets or liquid-stored platelets in response to ristocetin as another measurement of GPIb functional capacity. We found that approximately 15% of cryopreserved platelets lost surface-bound GPIb, while there was no measurable loss of GPIIB/IIIa during cryopreservation. The cryopreserved platelets also showed a significant decrease in aggregation to ristocetin, but no loss of response to the stronger agonist, thrombin. The loss of surface GPIb from cryopreserved platelets was modest in degree, approximately that reported for liquid-stored platelets, and does not seem great enough to account for the observed functional changes in aggregation and adhesion.