To achieve a rapid assay for ABO blood grouping using a latex reagent, two latex reagents were produced, one of which combined with mouse monoclonal immunoglobulin M (IgM) isolated from commercial ABO blood grouping reagent, and the other of which combined with its F(ab')2 fragment prepared by cold pepsin digestion. The latex reagent adsorbing the F(ab')2 fragment was able to detect the 1000-fold diluted saliva and provided much better sensitivity than that of IgM. This suggests that the difference in sensitivity between the two latex reagents is responsible for adsorption orientation of the antigen site on the latex particles. The new assay successfully completed the ABO blood grouping of cigarette ends within 30 min.