Equine immunoglobulins (eqIg) andits fragments, F(ab’)2and F(ab’), are normally employed as immunopharmaceuticals for several diseases, like treatment of snake bits and neutralizing bacterial toxins. However, it can induce serious side effects in a subset of patients, spe-cifically hypersensitivity from their immune response against whole eqIg contaminants. Therefore, the important antigenic determinants in eq-uine heavy chain IgG3(eqIgG3) were defined by a library of overlapping 15-mer peptides that covered the entire amino acid sequence (354 residues) and were immobilized onto a membrane.Screening with rabbit sera identified eleven distinct epitopes in eqIgG3 that were recog-nized by rabbit antibodies. The performance of an antisera against the most prominent epitope was analyzed by ELISA and a competitive assay. The results suggest an effective antisera to detect whole, undigested eqIg within equine sera preparations for minimizing adverse side effects and support this methodology for producing targeted antibodies.