A complement consumption assay was employed to test the consumption of guinea pig complement (GPC) by different sonicate and lipopolysaccharide (LPS) preparations obtained fromFusobacterium polymorphum.The consumption of normal GPC was observed at 37 C and varied depending upon the preparation used. All LPS preparations showed minimal activity at the concentrations employed in the assay. The observed anticomplementary activity was less potent at 4 C than at 37 C. Similar anticomplementary patterns were observed when inulin, a recognized activator of the alternate complement pathway, was tested in the complement consumption assay. The reactions of a sonicate preparation ofF. polymorphumwith GPC and hyperimmune rabbit antisera were not affected by the lower temperatures. The addition of heat treated (56 C and 30 min) GPC did not enhance the complement consumption displayed by a sonicate ofF. polymorphum.It is therefore suggested that this organism is capable of alternate pathway activity in GPC. Further tests revealed that the factors associated with the observed anticomplementary activity were resistant to the effects of heat (100 C for 30 min) and were located in the cell wall fractions of the sonicates. The cell walls also displayed endotoxic activity in aLimuluslysate assay. Because of these findings and the consistency of data produced by the cell walls at 37 C, it is concluded that the use of a cell wall preparation might be preferable to LPS in future investigations of this organism with the complement system.