The cytotoxic activities of culture supernates, crude cell extracts and cell-wall materials of Bacteroides gingivalis were investigated in vitro. Each component was cytotoxic to Vero cells and, to a lesser extent, Wi 38 cells. The cytotoxic agents had similar effects on the cell lines to butyric acid, propionic acid and a partially-purified trypsin-like protein extracted from a clinical isolate of B. gingivalis; the effects were eliminated by heat. Cytotoxic materials obtained from young cultures were more susceptible to heat than those from older cultures. The heat-labile substance inside and outside the bacterial cell in young cultures of B. gingivalis may contribute to its overall cytotoxic activity.