The assumption that immune and normal alloantibodies of the ABO system could be distinguished by partial neutralization with soluble specific blood group substance (Witebsky test) has been reexamined. The following results were presented: 1. ABO alloantibodies in both normal and immune sera as well as their IgG and IgM preparations were inhibited by their homologous soluble specific blood group substances. 2. Anti-A and anti-B immune antibodies as well as normal antibodies of the IgG class were found to be strong hemagglutinins in a saline medium; therefore they have to be called "complete" hemagglutinins as have anti-A and anti-B antibodies of the class IgM too. 3. ABO alloantibodies in both IgG and IgM preparations were able to form precipitation pellets with their homologous soluble specific blood group substances. IgM revealed a stronger precipitation power as IgG. 4. Agglutination reaction in saline was inhibited by the 50-200-fold group substance concentration needed for a optimal precipitation reaction, whereas agglutination of enzyme treated erythrocytes or red cells tested with antiglobulin (Coombs) sera was inhibited by a 20,000-80,000-fold concentration of the blood group substances. 5. Soluble antigen-antibody complexes, prepared from solubilized precipitates or from Witebsky test mixtures using chromatography, ultracentrifuge or ultrafiltration for separation were able to agglutinate erythrocytes in the antiglobulin or papain test. Following conclusions were drawn: A. Soluble antigen-antibody complexes are the main component leading to a positive Witebsky test. B. The mechanism of the Witebsky test as it has to be assumed in respect to our findings do not allow the distinguish immune and normal alloantibodies resp. IgG and IgM alloagglutinins in the ABO system.