This invention relates to an antitumor agent containing Lactobacillus casei YIT 9018 (Deposit No.  FERM-P 4751) as an effective ingredient.  This strain is deposited at Fermentation Research Institute, Government Industrial Research, Ministry of International Trade and Industry, Japan. 
While the antitumor activity of a live streptococcal preparation (Japan Tokkyo Koho, 6690, 1968) or an extract from lactobacilli (Japan Tokkyo Koho, 28558, 1970) has been reported, it has not been known that the heat-killed whole cell preparation has an antitumor activity.