Immunoglobulins isolated from egg yolk (IgYs) are a convenient source of polyclonal antibodies. Their purification is simple and the yields important (50 mg immunoglobulins/egg). Nevertheless their biochemical characteristics are different from those of rabbit antibodies generally used for the study of molecular defects of plasmatic and platelet proteins. Provided standardization is achieved, IgYs can be used for immunoelectrophoresis, immunoprecipitation or immunoblotting assays. We describe here the electrophoretic and immunoblotting conditions employed to explore human plasmatic and platelet von Willebrand factor (vWF) and human platelet GP Ilb-IHa using IgYs. These two proteins are involved in primary hemostasis and their absence or abnormality is responsible for hereditary bleeding disorders. The methods were applied to the characterization of patients with vWF or GP Ilb-IIIa defects and compared to classical mammalian IgG immunoelectrophoretic techniques. Results were further confirmed by flow cytometric analysis.