Amino acids buried in the hydrophobic interior of a protein with polar side chain atoms, which are not involved in hydrogen bonding or electrostatic interactions, have an adverse effect on protein stability. Replacing such residues by hydrophobic ones may render a protein more stable. Asparagine 241, which is buried in the neutral protease of Bacillus stearothermophilus, was replaced by leucine by site-directed mutagenesis. This mutation increased the stability of the protein by 0.7 +/- 0.1 degree.