The complete amino-acid sequence of a neutral proteinase, produced by Bacillus cereus, was determined by protein sequencing. The neutral proteinase consists of 317 amino-acid residues. The primary structure is 70% homologous to thermolysin, a thermostable neutral proteinase and 45% homologous to Bacillus subtilis neutral proteinase. The zinc-binding site and the hydrophobic pocket of the active site are highly similar in all three proteinases. B. cereus neutral proteinase which is 20 degrees C less thermostable (60 degrees C) than thermolysin (80 degrees C) shows only minor differences in calcium binding sites and salt bridges compared to thermolysin (known from its X-ray diffraction analysis), whereas B. subtilis neutral proteinase (50 degrees C) differs considerably. Therefore it was assumed that the difference in thermostability between B. cereus neutral proteinase and thermolysin is not caused by different metal binding properties, or differences in the active site, but by changes within the rest of the molecule. Calculation of secondary structure potentials according to Chou & Fasman, hydrophobicity and bulkiness of the different structural elements and preferred cold----hot amino-acid residue exchanges indicated, that the thermostability of thermolysin compared to B. cereus neutral proteinase is caused by small effects contributed by numerous amino-acid exchanges distributed over the whole molecule, resulting in increased hydrophobicity of beta-pleated sheet and higher bulkiness of alpha-helical regions.