The 247-260 and 289-299 alpha-helices of Bacillus subtilis neutral protease have a lysine in their N-terminal turn. These lysines were replaced by Ser or Asp in order to improve electrostatic interactions with the alpha-helix dipole. After replacing Lys by Ser at positions 249 or 290, the thermostability of the enzyme was increased by 0.3 and 1.0-degrees-C, respectively. The Asp249 and Asp290 mutants exhibited a stabilization of 0.6 and 1.2-degrees-C, respectively. The results show the feasibility of stabilizing enzymes by introducing favourable residues at the end of alpha-helices.