Increased levels of hypo-galactosylated immunoglobulin (Ig)A1 (HG-IgA1) in IgA nephropathy (IgAN) have been detected using a Helix aspersa agglutinin lectin enzyme-linked immunosorbent assay (ELISA). In this study, we developed monoclonal antibodies to evaluate the HG-IgA1 in IgA nephropathy, aiming to gain a more consistent and reproducible assay. As an analogue to the HG-IgA1 hinge region, a 19 mer synthetic peptide with five GalNAc (sHGP) residues at positions 4, 7, 9, 11 and 15 [VPST(GalNAc)PPT(GalNAc)PS(GalNAc)PS(GalNAc)TPPT (GalNAc)PSPS-NH2] was synthesized. Two monoclonal antibodies against sHGP (35A12 and 44H8) that reacted with human IgA were developed. Also, their reactivities to serum IgA from IgAN patients (n = 49), patients with other forms of kidney diseases (OKD, n = 48), and healthy controls (HC, n = 41) were evaluated using ELISA assays. The binding levels of the two monoclonal antibodies against serum IgA were significantly higher (all comparisons, p < 0.0001, Steel-Dwass non-parametric test) in IgAN patients compared to HC and OKD patients. In each individual, there was a close correlation of IgA binding levels between 35A12 and 44H8 (R(2) = 0.737). These results indicate that the monoclonal antibodies recognize similar epitopes in HG IgA1, which is found predominantly in IgAN patients. The developed antibodies are proposed as a clinically useful tool for IgAN screening.