Human blood platelets were stored in autologous plasma at 4 degrees C or 22 degrees C and their surface changes were probed with three lectins--wheat germ agglutinin, lentil lectin and concanavalin A. Platelets stored at either temperature for different times showed increased sensitivity to lectins. Lectins which were nonagglutinating to fresh platelets readily agglutinated stored platelets. The platelets stored for 24 h or longer lost their ability to respond to thrombin but demonstrated enhanced aggregation with wheat germ agglutinin. Surface labelling experiments revealed progressive loss of a glycoprotein of Mr 150,000 (GPIb) together with the appearance of components with Mr 69,000, 60,000 and 25,000 respectively. New high molecular weight glycoproteins were detected only in stored platelets. These findings illustrate the usefulness of lectins for the detection of altered expression of surface glycoconjugates which may be a factor in storage related dysfunction of platelets.