A cysteine proteinase inhibitor has been purified from immature fruit of Malus domestica (var. Royal Gala). The Mr of this apple cystatin is estimated to be 19600 by MALDI-TOF mass spectrometry, 11200 by SDS-PAGE and 11000 by gel filtration. It is a relatively strong inhibitor of papain with a Ki value of 0.10 nM and also inhibits ficin and bromelain but not cathepsin B. An amino acid sequence was obtained from a peptide produced by trypsin digestion of the inhibitor. Comparison with other plant sequences shows a high degree of homology with other phytocystatins. As the single cysteine proteinase inhibitor detectable in immature apple fruit (4–7 mm diameter), levels of 72.2 pmol/g FW were determined. In larger fruit (up to 15 mm diameter) significantly less inhibitor was present (5.8 pmol/g FW). Given these low levels, it is postulated that this inhibitor has an endogenous role in apple fruit development rather than one of protection against pest or microbial attack.