Papain exists in molten globule (MG) state at pH 2.0 and in this state protein tends to aggregate in the presence of lower concentrations of guanidine hydrochloride (GuHC1). Such aggregation is prevented if a low concentration of urea is also present in the buffer; in addition, stabilization of the protein is also induced. Intrinsic fluorescence properties of papain as well as ANS binding suggest significant changes in the structure of papain, in the presence of urea with the absence of major changes in the secondary structure of the protein. The GuHCl- and temperature-induced unfolding of papain, in the presence of urea, indicates stabilization of the protein as seen from the higher transition midpoints, when monitored by fluorescence and circular dichroism. (CD). However, a similar phenomenon is not seen under neutral conditions in the presence of urea either at low or high concentrations. The utility of prevention of aggregation by urea is also discussed. (C) 2002 Elsevier Science (USA).