The activity and conformational changes of ficin (EC 3.4.22.3), a cysteine protease from Ficus carica have been investigated during the denaturation by urea and guanidine hydrochloride (GuHCl). The denaturation of ficin was followed by activity measurements, fluorescence and circular dichroism (CD) spectroscopic studies. The enzyme activity decreased significantly at low concentration of both urea and GuHCl before unfolding of the enzyme molecule. The enzyme molecule was resistant for unfolding by urea under neutral conditions even at higher concentrations. However, the protein is susceptible to unfolding by urea at lower pH and transition follows a cooperative two-state rule with increasing concentration of urea. On the other hand, ficin molecule loses its complete structure in presence of 4 M GuHCl under neutral conditions. The GuHCl-induced unfolding occurs in a simple two-state cooperative process. These results indicate the differential structural stability and fragility of active site of the enzyme towards denaturation by urea and GuHCl. (C) 2010 Elsevier Ltd. All rights reserved.