The region between the amino acids 31-46 was previously identified as being first exposed during thermal unfolding of bovine pancreatic ribonuclease A (RNase). The exchange of one amino acid (Leu35-->Ser) in this unfolded region of RNase is shown to have a dramatic destabilizing effect (DeltaT(m) = 9 C). Antibodies raised against a peptide corresponding to the sequence of the labile region, S32-V43, of RNase were effective in stabilizing L35S-RNase against thermal inactivation (65degreesC for 2 h) and surpassed the stabilization effect of antiRNase antibodies. An 11% contribution to the stabilizing effect of antiRNase antibodies resulted from antibodies recognizing the unfolding region of the enzyme.