Titres of IgG anti-A or -B in the serum of 64 blood group O women with an obstetrical history of possible ABO blood group incompatibility were examined. Four different 2-mercapto-ethanol (2-ME) techniques varying in time from 3-16 h were evaluated. No definite differences in anti-A or -B titres were observed. When AB substances were used, however, the anti-A or -B titres were on the average 2-3 titration steps lower than in the 2-ME techniques, if demonstrable at all. Besides agglutinating IgG anti-A or -B, haemolysins IgG anti-A or -B could be demonstrated in part of the sera. In 8 sera, the IgG and IgM fractions were separated on sephadex G-200 and treated with 2-ME and AB substances. In the IgG fraction, no definite differences in complete, incomplete and haemolysin anti-A or -B titres were found before and after treatment with 2-ME. Using AB substances, no complete anti-A or -B was found anymore, while the incomplete titres were on the average 2-3 titration steps lower. The anti-A or -B antibody activity of the IgM fraction was completely destroyed by 2-ME and AB substances. From this study we concluded that IgG anti-A or -B could be determined by 2-ME within a few hours and that this technique is much more reliable than the partial neutralization test with AB substances.