Antibodies with different recognition properties are of considerable importance for antibody pair screening to successfully establish sandwich immunoassays, significantly affecting the determination of minimum distance of the two epitopes. To obtain antibodies against MEL and NIA epitopes with recognition properties and as much diversity as possible, a hapten spacer arm between the epitope and carrier protein should be first considered, since the full exposure of the target analyte to the immune system is critical to induce highly specific antibodies. Thus, two sets of novel haptens were designed and named M1-M6 and N1-N6 (Fig. and). These haptens were characterized by the equably increased spacer arm length of two methylenes to produce antibody populations with various recognition properties toward MEL or NIA epitopes.