Investigations to demonstrate irregular erythrocyte antibodies showed an increase in the number of sensitizations due to blood transfusions as well as an increase of the antibodies anti-E and anti-Kell. A reorganization of the blood transfusion system should be considered. It seems necessary to detect patients with irregular antibodies as early as possible. This should be done by follow-up investigations after blood transfusion and by antibody screening tests before transfusion. Blood groups estimated previously should not be relied upon during subsequent hospital admissions.