The ICP family of cysteine peptidase (CP) inhibitors (founder: chagasin) share no sequence similarity with known CP inhibitors e.g. the cystatins. ICPs are the first CP inhibitors to be identified in prokaryotes. ICPs inhibit clan CA family C1 CPs with Kis in the sub-nM range. The role of the ICPs remains uncertain: in the protozoan species where they were first discovered they may have a protective role against endogenous CPs such as cruzain and CPB, or they may be directed against host CPs (Besteiro et al. (2004)). The discovery of an ICP in the prokaryote Pseudomonas aeruginosa, whose genome contains no clan CA family C1 CP genes, supports the latter hypothesis (Sanderson et al. (2003)). All L.mexicana ICP polypeptide backbone resonances were assigned, with the exception of the N-terminal amides of the two fragments (residues -2 and 6). Assignment of non-labile amino acid sidechain positions is essentially complete. Assignments deposited with the BMRB accession number 6794.