ABO blood group incompatibility is of major concern because the antibodies frequently cause intravascular destruction leading to the most clinically severe complications. Irregular erythrocyte antibodies seldom cause intravascular destruction. However, when the antibodies bind complement and are present in high concentration the extravascular destruction occurs predominantly in the liver and may cause a clinically severe reaction with hemoglobinemia, shock and DIC. Those IgG class antibodies which do not bind complement cause a spleen-mediated, more or less drastically shortened life span of the red cells with unpleasant but usually not life threatening consequences for the patient. The transfusion reaction may occur with a delay of several days. Compatibility testing may consist of either a cross-match performed in such a way that both ABO incompatibility and irregular antibodies are detected, or a type-and-screen procedure. In the latter a computerized delivery control checking the patient's previous serological history and the donor/recipient compatibility is a recent improvement and rationalization. In an acute situation a reduction of the normal security measures may be necessary. The clinician responsible for the patient must know the time needed for carrying out an acute blood group determination and compatibility test in the hospital and must give instructions about what normal security measures that should be refrained from. Blood group incompatible transfusions have nowadays a much better prognosis than earlier, provided a rapid diagnosis can be made and appropriate treatment can be started promptly. Formation of antibodies against leukocyte/thrombocyte antigens is a frequent consequence of transfusion which, however, can be prevented by modern blood component therapy.(ABSTRACT TRUNCATED AT 250 WORDS)