Hemipterans (bugs) and most coleopterans (beetles) are the only insects that have cysteine proteinase as a digestive enzyme. Detailed knowledge on this enzyme is needed before using it as target for new insect control procedure. The midgut of D. peruvianus was divided into three sections (V₁, V₂ and V₃). The region V₂ was selected as a source of material for purifying a cysteine proteinase because it contains most of the activity of that proteinase. After several attempts to purify this proteinase, an effective process was developed that avoid autolysis with methyl methanethiosulfonate (MMTS), a sulfhydryl-reactive and reversible sulfonating reagent for thiol-containing molecules. It converts sulfhydryl groups on cysteine side chains into dithiomethane (–S-S-CH₃). The V₂ of D. peruvianus was homogenized in a 1 mM MMTS solution, centrifuged at 20 000g at 4°C for 30 minutes. The supernatant was applied on to a gel filtration column (Superose 12 HR) using a FPLC system. The active fractions were pooled and passed through an affinity column (Sepharose Arg) with the aid of an AKTA prime plus system. During all chromatographic steps the protein was monitored by its activity against a fluorogenic synthetic peptide, Z-FR-MCA (carbobenzoxy-Phe-Arg-7 amido 4 methyl coumarin), in 0.1 M MES buffer pH 6.0. The purified protein presents a molecular mass of approximately  30 kDa (SDS-PAGE) and recovery of 50%.