In the present study, some antitumor mechanisms of Eubacterium lentum (TYH-11) and bacterial components having antitumor effects were investigated. E.lentum induced maximum NK cell activity in C3H/He mice on day 1 after injection (90.6% against 33.9% of control at E:T ratio 50:1) and the activity was kept at a level of 48.6% on day 7. Tumoricidal peritoneal macrophages were induced 9 days after E.lentum injection into BALB/c mice (56.2% against 10.1% control at E:T ratio 10:1). Tumoricidal macrophage activity persisted at the same level for at least 11 days. Cytotoxic T lymphocyte (CTL) activity was induced only in tumor bearing mice treated with E.lentum, 4 weeks after tumor inoculation. Antitumor activity was observed in the cell wall (CW) and membrane fractions (CM) of E.lentum. CW induced NK cell activity; the activity was transient while the kinetics of NK activity by CM showed 2 peaks, on day 1 and day 7. Tumoricidal macrophages were induced by CW and the activity level was the same as that induced by whole body, while that induced by CM was at a lower level. Neither CW nor CM induced CTL in tumor bearing mice.