A decrease of platelet agglutination induced by ristocetin has been described in cirrhotic patients. In order to investigate the relationship of such phenomenon with a putative defect on platelet membrane glycoprotein Ib, we have studied the quantitative and functional status of Gp Ib in eleven severe alcoholic cirrhotic patients, and the ability of their formalin-fixed platelets to agglutinate in the presence of normal plasma plus ristocetin. Interestingly, we found a significant decrease of immunoreactive GP Ib molecules (9,978 +/- 1,534 vs. 17,064 +/- 404 molecules per platelet) and ristocetin-dependent binding of vWF (9,113 +/- 1,338 vs. 13,992 +/- 1,968 molecules per platelet) (P < 0.01) in comparison to the levels found in a control group of healthy subjects. Immunoblotting analysis of platelet lysates confirmed the reduction of GP Ib level in cirrhotic patients, but showed no modification on the precipitation pattern of this glycoprotein. The ristocetin-induced platelet agglutination (light transmission %) was also significantly lower in patients with cirrhosis (48 +/- 7.6 vs. 92 +/- 3.6, P < 0.01), and correlated with the binding of normal vWF (r = 0.863, P = 0.0013). Patients with bleeding times longer than 7 min and/or clinical history of bleeding episodes showed the lowest values of platelet agglutination and GP Ib level. In conclusion, our present results indicate that liver cirrhosis is associated with a relevant decrease of functional GP Ib molecules on the platelet surface. This reduction might be related to the prolongation of bleeding time and to the bleeding diathesis observed in cirrhotic patients.