Isoelectric focusing was used to characterize proteolytic enzymes in homogenate and excretory-secretory preparations of the larvae of L. cuprina, the sheep blowfly. Zymogram overlays showed that the larvae produce a number of highly active proteases which have a wide range of isoelectric points and molecular weights. The alkaline and neutral pI proteases were inhibited by phenylmethyl-sulfonylfluoride, leupeptin and aprotinin; this indicated the presence of serine in the active site. Pepstatin and the metal chelating agent ethylenediaminetetraacetic acid had no effect on the activity of any of the proteases. Optimal pH for activity of the proteases was between 7 and 8. In addition, the proteases were found to be heat labile. Digestion of collagen fibrils confirmed the existence of collagenolytic activity in the excretory-secretory enzyme preparations. It is suggested that these enzymes may be involved in the nutrition of the larvae and in the pathogenesis of the lesion on the skin.