The leader proteinase (L^pro) of foot-and-mouth disease virus (FMDV) is an animal   pathogen of global importance. L^pro initially cleaves itself from the nascent viral polyprotein, and   them   cleaves the host eukaryotic initiation factor (eIF) 4GI and 4GII. Although, a very rapid inhibition of host cell protein synthesis occurs, viral translation is unaffected (Glaser,W et al. 2001).  FMDV L^pro is a papain-like viral cysteine protease. We have synthesized a serie of fluorescent peptides derivated from the sequence of self-processing and eukaryotic initiation factor eIF4G. Kinetic data on the cleavage sites of L^pro on the viral polyprotein eIF4GI isoform suggest that the enzyme requires a basic residue at either the P1 or P´1 position with a hydrophobic residue at P2. However, for eIF4GII we have observed that a basic residue adjacent at P1 is not essential to occur the cleavage. Preliminary experiments to determine optimal assay conditions using the fluorescent peptides showed that L^pro was sensitive to monovalent cations. Approximately 50% of the enzymatic activity was lost when raising the NaCl (0 to 20 mM) or the sodium citrate (0 to 10 mM) concentration. A detailed pH-profile analysis of L^pro activity was performed, which showed to be dependent on the salt and buffer nature and concentration.