In order to investigate the nature of the protein layer deposited on hemodialysis membranes we developed a direct immunohistochemical technique using fluoresceinated antibodies to plasma membranes. Fifteen patients on regular dialytic treatment were dialyzed with CU, HE, PAN, PS and PMMA and the dialyzers analyzed at the end of a standard dialytic session. Snap frozen sections of hollow fiber devices were treated with flourescein-isothiocyante conjugated antibodies for IgG, IgA, IgM C3c, fibrinogen, factor VIII, factor XIIIa-s, antithrombin III, fibrinogen degradation products (PDF) and plasminogen. Protein deposits were evaluated by a quantitative criteria, which evaluates the intensity of fluorescence and the area of the capillary wall occupied by this fluorescence by using an image analysis software. The coagulation cascade is activated by all membranes and similar deposits of these proteins were revealed, whereas important differences in C3c deposition were found.