Saporin, a monomeric protein extracted from the seeds of Saponaria officinalis, is an enzyme capable of specific depurination of the eukaryotic ribosomes. Because of its toxicity, saporin proved useful for the synthesis of immunotoxins, chimeric conjugates of a toxin and an antibody specifically directed against cancer cells or other targets. In this paper we report a study of the structural properties of saporin in the presence of denaturing agents and/or proteolytic enzymes. We found that saporin is extremely resistant to denaturation by urea or guanidine (up to 4 M), even at relatively high temperature (up to 55 degrees C). Moreover a structural change detected as a reduction of the fluorescence emission of the single Trp residue is reversible and is not paralleled by changes of the far UV CD spectrum, suggesting that even under harsh experimental conditions unfolding is limited. In good agreement with these results, guanidine-treated saporin is not attacked by proteolytic enzymes. The remarkable resistance of saporin to denaturation and proteolysis suggests this protein as an ideal candidate for biotechnological applications.