Protein inhibitors capable of inhibiting BCP (Bombyx cysteine proteinase) were found in the larval-pupal hemolymph of Bombyx mori. Two forms of the inhibitors, named BCPI (BCP inhibitor) a and BCPI b, were purified from the pupal hemolymph by heat treatment and column chromatographies on CMcellulose, Toyopearl HW-50, Phenyl-Sepharose, and Mono Q. Purified BCPI b gave a single protein band with a molecular mass of 10,500 daltons on SDS-PAGE. BCPI a is mostly composed of the same molecular mass protein as BCPI b. Both forms were inhibitory towards other cysteine proteinases such as cathepsins L,B and papain but had no effects on trypsin and pepsin. Both forms inhibited the processing of the enzymatically inactive proform of BCP (pro-BCP) to the activated mature BCP. BCPI a and BCPI b shared many other features such as molecular mass determined by gel filtration, antigenicity, and HPLC profiles. NH 2 -terminal amino acid sequencing of the purified inhibitors revealed that three amino acid residues were different in the BCPI a and BCPI b sequences, all others being identical. The hemolymph BCP inhibitor increased activity approximately four-to fivefold at the time of spinning and maintained this level of activity during pupation. Arch. Insect Biochem. Physiol. 41:119-129, 1999.