Résumé : The immuno-haemolytic risk of the blood transfusions is directly related to the immunogenic character of the blood groups systems. According to data’s of national haemovigilance network, the immunological risk is estimated at 1/8900 transfused blood products. The risk relating to incompatibility ABO accounts for of them approximately 7 % and that related to the other blood systems contributes to it to a total value of 17 %. The analysis of these events reveals that they are primarily related to human failures. The immuno-haematologic tests define the antigens and detect the antibodies present in the blood product and the patient blood. These tests make it possible to select the units not comprising antigens corresponding to the antibodies of the patient and not comprising antibodies corresponding antigens of the patient. The pretransfusion tests in usual situations are ABO-RH1(D) group, RH-KEL1 phenotype and antibody screen. Other tests are necessary in neo-natal transfusion and haemolytic transfusion reaction. Their analytical realization and their exploitation represent critical stages of the transfusional process. Their reliability rests on technical criteria defined by the regulation, on the automation and the data-processing transfers of results. The phases of blood samples collection and identification of the patient, generally implied in the immuno-haemolytic accidents, must be the subject of the greatest attention.