PURPOSE: To obtain a drug preparation produced from Bacillus natto and having antitumor activity, by culturing Bacillus natto in a medium containing water- extract of soybean or contacting Bacillus natto with an antibiotic substance or heating Bacillus natto at a temperature above a protein-denaturation temperature and using the obtained cells of Bacillus natto as active component.

CONSTITUTION: Bacillus natto [bacterial strain used in the production of NATTO (fermented soybean)] is cultured in a soybean extraction liquid produced by extracting soybean with hot water at about ≥90°C and the bacterial cells are collected from the culture product. The cells are suspended in a liquid medium such as Bernheimer base medium, added with sufficient amount of an antibiotic substance having β-lactam ring (e.g. penicillin or cephalosporin antibiotic substance) and maintained for a short time. As an alternative method, the bacterial cells or a suspension of cells are heated at 70W100°C (i.e. above the protein-denaturation temperature) for 10 W several tens min and the treated cells are collected.