Potential antitumor activity of Lactobacillus plantarum cytoplasm was examined using F9 teratocarcinoma-bearing BALB/C mice. The cytoplasmic fraction of L. plantarum was separated by sonication followed by ultracentrifugation. The fraction at a dose of 100 or 200 mg/kg/day was orally administered for 7 consecutive days before or after tumor inoculation to 16 mice. As a control, heat-killed whole cell was used at a dose of 100 mg/kg/day. Upon oral administrations of both the cytoplasm and heat-killed whole cell, when performed after and before tumor inoculation, the survival of F9-bearing mice prolonged more effectively. Administration of the cytoplasm after tumor inoculation extended the average survival days by 30 and $40\%$ at daily dosages of 100 and 200 mg/kg/day, respectively. This result suggests that the cytoplasmic fraction of L. plantarum has strong antitumor activity against mouse F9 teratocarcinoma in vivo.