Pretreatment of platelets with chymotrypsin dose-dependently decreased glycoprotein (GP)-Ib amounts as measured by SDS-PAGE, ristocetin-induced agglutination and platelet electrophoretic mobility (EPM). Decrease in platelet EPM in response to 0.75 mg/ml ristocetin alone were 7.0 +/- 2.3 and 6.8 +/- 4.3% (M +/- S.E., n = 6) for control and chymotrypsin-treated platelets, respectively (p greater than 0.2). Von Willebrand factor (vWF) alone had no effect on platelet EPM. However, in the presence of 0.75 mg/ml ristocetin, added vWF (2.9 micrograms/ml) caused a further 6.3 +/- 3.8% decrease in control platelet EPM, but caused no significant decrease in the enzyme-treated platelets (p less than 0.05). In the presence of 0.3 mg/ml ristocetin, added vWF (2.9-14.5 micrograms/ml) caused a small but significant decrease in control platelet EPM, but caused no significant decrease in the enzyme-treated platelets. These findings suggested that the GP-Ib carrying negative charge decreased by binding of vWF might facilitate a mutual approach of the GP-Ib molecules and bridge formation by vWF between different platelets.