The presence of whole horse IgG in therapeutic snake antivenom preparations of high purity is a contamination that can cause IgE-mediated allergic reactions in patients. In this study, the immunodominant IgE and IgG-binding epitopes in horse heavy chain IgG3 were mapped using arrays of overlapping peptides synthesized directly onto activated cellulose membranes. Pooled human sera from patients with and without horse antivenom allergies were used to probe the membrane. We have demonstrated that, for both cases, individuals produce antibodies to epitopes of sequential amino acids of horse heavy chain IgG3, although the signal strength and specificity appear to be distinct between the two groups of patients. A single region was found to contain the dominant allergic IgE epitope. The critical residues involved in the binding of human IgE to the epitope were determined to include four hydrophobic amino acids followed by polar and charged residues that formed a coil structure. This is the first study to describe the specific amino acid sequences involved with the immune recognition of human IgG and IgE to horse antivenom.