Abstract: A method of, and support and kit for, determining blood groups by the solid-phase procedure with blood-group specific immunoglobulin-M antibodies. The support is prepared by applying to its surface a solution of the immunoglobulin-M antibodies and simultaneously applying to the surface of the support a suspension of erythrocytes that is being tested, or drying the surface of the support to form a ready-to-use reagent, and subsequently contacting the surface with a suspension of erythrocytes, and determining the blood group just after the contact occurs, the concentration of antibodies being just sufficient to create an irreversible or immobilized coating of antibody molecules on the surface that can specifically attach erythrocytes but insufficient to induce an agglutination reaction.