To elucidate the biochemical activation mechanism of the insect pro-phenoloxidase (pro-PO) system, we purified a 45-kDa protein to homogeneity from the hemolymph of Tenebrio molitor(mealworm) larvae, and cloned its cDNA. The overall structure of the 45-kDa protein is similar to Drosophilamasquerade serine proteinase homologue, which isanessential component inDrosophilamuscledevelopment.