Bursicon is a polypeptide obligatory for the sclerotization of the newly formed insect cuticle after each molt during larval development. Our study reveals for the first time the basic biochemical characteristics of bursicon of the mealworm beetle Tenebrio molitor. Tenebrio molitor larvae (20 kg; approximately 290,000 animals) were used to establish the purification scheme as outlined. The purification scheme includes 4 prepurification steps, one gel filtration step and 3 HPLC steps and results in an approximately 108,000-fold purification. SDS gel electrophoresis of this material reveals two protein bands at about 30 and 45 kDa. Bioactive bursicon was eluted only from the region containing the 30 kDa protein. Two-dimensional SDS-PAGE show only two spots at 30 kDa; bursicon activity was eluted only from the spot at 30 kDa/5.65 pH. The comparison of the apparent molecular weight of bursicon under reducing and non-reducing conditions indicates that it is a single-chain molecule.