IgM is the predominant immunoglobulin (Ig) class of anti-A or anti-B Abs produced by group B or A individuals, while IgG is the dominant class of anti-A and anti-B Abs in group O serum. In the hemagglutination (HA) method, the room temperature (RT) incubation technique and the indirect antiglobulin test (IAT) have been interpreted as the methods detecting IgM and IgG, respectively [1, 2, 3]. However, both IgM and IgG of ABO Ab can agglutinate RBCs at RT (20-24℃) or below and efficiently activate the complement at 37℃ [4]. Therefore titers using RT techniques or IAT on dithiothreitol (DTT) untreated samples may be more reflective of the mixed concentration of IgM and IgG of ABO Ab.