Immunosera for envenoming treatment are the most traditional biotechnological products, since their production began in XIX century. The European Pharmacopoeia (EP) establishes that immunosera must be pure intact immunoglobulin or F(ab)´₂ fragment, with its purity evaluated by High Performance Liquid Chromatography (HPLC) and SDS PAGE electrophoresis, since contaminant proteins from animal plasma are responsible for sera adverse reactions. In Brazil, F(ab)´₂ is obtained from immunized horses, and according to the Brazilian Pharmacopoeia (BP), a non-qualitative method, the protein content assay, is used for determining sera purity. In this work, we have evaluated the purity of Brazilian antitoxins by SDS PAGE analyses in non-reducing conditions. It was shown that in many batches of the product, proteins with molecular weights different from those expected to F(ab)´₂ were present. To characterize these contaminant proteins, samples were submitted to two dimensional gel electrophoresis. Protein spots were submitted to trypic digestion and MALDI-TOF mass spectrometry analyses. Most spots were isoforms of F(ab)´₂. Other spots were Fc fragment, horse albumin, pepsin and intact immunoglobulin, which were not removed during the production process. These results are in accordance with the incidence of adverse effects in immusera used in Brazil. We are now developing an immune affinity method to improve protein purity analyses of Brazilian antitoxins.