The unfolding of the blue-copper protein azurin from Pseudomonas aeruginosa by guanidine hydrochloride, under nonreducing conditions, has been studied by fluorescence techniques and circular dichroism. The denaturation transition may be fitted by a simple two-state model. The total free energy change from the native to the unfolded state was 9.4 +/- 0.4 kcal.mol-1, while a lower value (6.4 +/- 0.4 kcal.mol-1) was obtained for the metal depleted enzyme (apo-azurin) suggesting that the copper atom plays an important stabilization role. Azurin and apo-azurin were practically unaffected by hydrostatic pressure up to 3000 bar. Site-directed mutagenesis has been used to destabilize the hydrophobic core of azurin. In particular either hydrophobic residue Ile7 or Phe110 has been substituted with a serine. The free energy change of unfolding by guanidinium hydrochloride, resulted to be 5.8 +/- 0.3 kcal.mol-1 and 4.8 +/- 0.3 kcal.mol-1 for Ile7Ser and Phe110Ser, respectively, showing that both mutants are much less stable than the wild-type protein. The mutated apoproteins could be reversible denatured even by high pressure, as demonstrated by steady-state fluorescence measurements. The change in volume associated to the pressure-induced unfolding was estimated to be -24 mL.mol-1 for Ile7Ser and -55 mL.mol-1 for Phe110Ser. These results show that the tight packing of the hydrophobic residues that characterize the inner structure of azurin is fundamental for the protein stability. This suggests that the proper assembly of the hydrophobic core is one of the earliest and most crucial event in the folding process, bearing important implication for de novo design of proteins.