The microheterogeneity and polymorphism of platelet membrane glycoproteins IIb and IIIa from hypercholesterolemic type IIA patients and from normolipidemic controls were compared by isoelectric focusing followed by immunological detection. Their size and the extent of glycosylation were investigated by polyacrylamide gel electrophoresis and carbohydrate affinity stain. No qualitative or quantitative differences could be detected, either within each test group or between the two groups. This finding rules out the hypothesis that altered molecular forms of the glycoproteins forming the fibrinogen receptor might be involved in the pathogenesis of platelet hyperaggregability in hypercholesterolemic patients.