The present invention is a bacteriolytic agent containing a cationic surfactant (A) of which a counteranion is an acid having a pKa (25.degree. C.) of 0 to 10. As the counterion, a carboxylate anion is preferable. As the cationic surfactant (A), a quaternary ammonium salt-type surfactant is preferable. Examples of the useful substance include a protein, an amino acid, a nucleic acid, an antibiotic, sugars or vitamins. The bacteriolytic agent of the present invention is excellent in a bacteriolytic power in a step of extracting a useful substance from a microorganism (useful substance producing bacterium etc.). In addition, denaturation of the useful substance during the step is small.