A fluorescence immunoassay method is used in the differentiation of A₁ and A₂ bloodstains. Anti-A antiserum is first absorbed onto the stain. The IgM content in the eluate is then quantified by commercial Immuno-Fluor kits. The binding difference of A₁ and A₂ cells is retained in stains. This difference and the quantitative evaluation method provide a potential basis for blood typing or subtyping application.