Patients with uraemia have abnormal platelet function that may be partially corrected by haemodialysis, cryoprecipitate or 1-desamino-8-d-arginine vasopressin (DDAVP). We studied the platelet von Willebrand factor receptor. glycoprotein Ib (GPIb). and plasma von Willebrand factor (vWF) in uraemic patients undergoing chronic haemodialysis. Using the slope of agglutination of formalin-fixed platelets as an index of response to ristocetin (with a constant amount of normal plasma as a source of vWF), we found the response of platelets from uraemic patients, both before (2·7± 1·5. n= 40) and after dialysis (1·2 ± 1·2, n= 40) to be significantly less than that for normal controls (14·1 ± 10·2. n= 20: P < 0·001). In addition, the agglutination response of platelets obtained after dialysis was less than that of platelets obtained before dialysis (P < 0·001). Immunoblotting demonstrated decreased or absent staining of glycocalicin, a subunit of GPIb, in platelet lysates from 25 patients. All platelet samples with reduced glycocalicin also had decreased responses to ristocetin. Tritium-labelled platelets from seven patients showed decreased labelling of a protein with an electrophoretic mobility equivalent to that of GPIb (140000 daltons). In addition, platelets with the lowest levels of surface GPIb, as demonstrated by flow cytometry. also had decreased ristocetin agglutination and decreased staining on immunoblot. Levels of von Willebrand factor antigen and ristocetin cofactor in plasma from 10 patients were generally within the normal range, although post-dialysis levels tended to be higher than pre-dialysis levels. The pre- and post-dialysis plasma vWF multimeric patterns were normal.