New cysteine protease inhibitors, named cathestatin A and B, have been discovered as metabolites of Penicillium citrinum. Cathestatins were found to be decarbamidoyl analogs of estatins and showed a specific inhibition for cysteine proteases. Cathestatins suppressed parathyroid hormone (PTH)-stimulated 45Ca release in organ cultures of chick embryonic calvaria.