The complete amino acid sequence of the inhibitor of cysteine proteinases from pineapple stem acetone powder was determined. The inhibitor consists of 52 amino acids and is composed of two polypeptide chains (41 and 11 amino acids) linked via disulphide bonds. It differs from already known sequences in one to four amino acids. Data from its amino acid sequence analysis clearly show that this inhibitor cannot be a member of the cystatin superfamily. The Ki values for papain, bromelain and cathepsin L were determined.