A blue biliprotein, insecticyanin (INS), has been purified from the last instar larval hemolymph of Agrius convolvuli by ultracentrifugation, Sephadex G-100 gel permeation chromatography, and preparative electrophoresis. The molecular mass of INS was estimated to be 26 kDa and the N-terminal sequence of INS revealed high similarity to that of Manduca sexta. Results of Western blotting and autoradiography indicated that INS is synthesized by the epidermis and released into the hemolymph. In contrast to the INS reported in other insects, Agrius convolvuli INS contained a small amount of lipid, predominately consisting of triacylglycerol. Subcellular localization of INS was determined using protein-A gold particles linked to secondary antibodies (anti-rabbit Ig). INS was heavily accumulated in the cytoplasmic inclusion body (CIB). CIBs showed a variety of shapes from rod to globule and generally surrounded the nucleus. They were mostly located near the basement membrane and especially abundant in the intersegmental membrane.