Potato tubers contain considerable amounts of inhibitors of cysteine proteinases. The majority of inhibitory activity is due to low-molecular mass inhibitors differing in isoelectric points. Three of them were obtained in homogenous form, namely PCPIs (potato cysteine proteinase inhibitors) 6.6 (Mr 25,000), 8.3 and 9.4 (both Mr 22,000). They all appeared to be single-chain proteins. The amino-acid composition and the N-terminal amino-acid sequences show that at least two of them are homologous proteins, but so far no homology to the inhibitors of the cystatin super-family or to any other sequenced potato proteins is apparent. PCPIs inhibit papain and human cathepsins B, H, and L. The inhibitors interact with enzymes in apparently equimolar fashion, the interaction is of the tight binding type with Ki values ranging from 10(-6)M to 10(-11)M.