The basic ABO blood group system discovered by Landsteiner in 1900 is still the most important blood group system in transfusion therapy. The presence of anti-A and anti-B isoagglutinins in blood which does not contain the corresponding antigen in the red cells represents one of the major hazards in blood transfusion therapy. While the majority of the so-called irregular blood group antibodies usually are not present in blood unless a stimulation has taken place either by pregnancy or transfusion, the anti-A and anti-B antibodies are present without this stimulation and will always remain a latent danger in blood transfusion therapy. For this reason the use of group O blood as so-called universal donor blood has always intrigued workers in the field of blood transfusion. When Witebsky and co-workers in 1941 published their paper on the "Preparation and Transfusion of Safe Universal Blood" it was hoped that the solution to the