Method for the demonstration of a specific complex formed by reacting an anti-antigen antibody of blood group or phenotype present in a solution, and a blood group or phenotype antigen carried by an erythrocyte, the said erythrocyte being bound to a magnetic particle, the reaction taking place in reactor with an open top and sealed base whose diameter decreases at least in the area close to the base in such a way that it forms an inclined wall being at least partially coated with an anti-immunoglobulin or with any other compound capable of binding to the antibody of the said formed complex, the said method consisting into several steps: a) prior to the contact between the magnetic particles suspension bearing the erythrocytes and the solution likely to contain anti-antigen antibodies from blood group or phenotype: - filling of the reactor with a viscous solution such that to cover up at least part of the inclined wall of the reactor, b) bring into contact, above the viscous solution contained in the reactor, of the solution containing or likely to contain the said antibody with the suspension of magnetic particles bearing the erythrocytes; c) incubation of the reactor for a given time necessary to the formation of the complex between the magnetic particles bearing the erythrocytes and the anti-antigen antibodies of blood group or phenotype likely to be contained in the solution and to specifically recognize the said antigens of blood group or phenotype carried by the erythrocytes; d) application of a magnetic field to the said reactor and stirring of the reactor, such that the magnetic particles are drawn towards the bottom and/or the inclined wall of the reactor; and e) reading with a naked eye and/or with any other appropriate reading system, of the image obtained at the bottom of the reactor and/or on the inclined wall of the reactor coated with the said anti-immunoglobulin or with any other compound capable of binding to the antibody, this obtained image making it possible to demonstrate the presence or not of the formation of a specific complex antibody/antigen of human group or phenotype, characterized in that the said magnetic particles bearing the erythrocytes are magnetic particles which have been previously coated with an anti-glycophorin A antibody (anti-GPA) and in that said erythrocyte is bound to said magnetic particle by an interaction of antibody/antigen type. a) prior to the contact between the magnetic particles suspension bearing the erythrocytes and the solution likely to contain anti-antigen antibodies from blood group or phenotype: - filling of the reactor with a viscous solution such that to cover up at least part of the inclined wall of the reactor, - filling of the reactor with a viscous solution such that to cover up at least part of the inclined wall of the reactor, b) bring into contact, above the viscous solution contained in the reactor, of the solution containing or likely to contain the said antibody with the suspension of magnetic particles bearing the erythrocytes; c) incubation of the reactor for a given time necessary to the formation of the complex between the magnetic particles bearing the erythrocytes and the anti-antigen antibodies of blood group or phenotype likely to be contained in the solution and to specifically recognize the said antigens of blood group or phenotype carried by the erythrocytes; d) application of a magnetic field to the said reactor and stirring of the reactor, such that the magnetic particles are drawn towards the bottom and/or the inclined wall of the reactor; and e) reading with a naked eye and/or with any other appropriate reading system, of the image obtained at the bottom of the reactor and/or on the inclined wall of the reactor coated with the said anti-immunoglobulin or with any other compound capable of binding to the antibody, this obtained image making it possible to demonstrate the presence or not of the formation of a specific complex antibody/antigen of human group or phenotype, characterized in that the said magnetic particles bearing the erythrocytes are magnetic particles which have been previously coated with an anti-glycophorin A antibody (anti-GPA) and in that said erythrocyte is bound to said magnetic particle by an interaction of antibody/antigen type.