Periodontitis is a bacterially induced chronic inflammatory disease and a major cause of tooth loss in the world. The tissue damage and alveolar bone resorption characteristic of the disease are believed to be due to a destructive innate host response to a pathogenic subgingival biofilm. Porphyromonas gingivalis, a Gram-negative bacterium, is a member of this mixed microbial community that has been designated an etiologic agent of periodontitis. The innate host response to lipopolysaccharide (LPS) obtained from P. gingivalis is unusual in that different studies have reported that it can be an agonist for Toll-like receptor (TLR) 2 as well as an antagonist or agonist for TLR4. In addition, human monocytes respond to this LPS by secreting a variety of different inflammatory mediators, while endothelial cells do not. We have examined highly purified preparations of P. gingivalis LPS and found that they activate both TLR2 combined with TLR1 and TLR4 in transiently transfected human embryonic kidney (HEK) 293 cells. We have further demonstrated that highly purified P. gingivalis LPS preparations contain at least 3 major different lipid A species. We speculate that P. gingivalis lipid A structural heterogeneity contributes to the unusual innate host response to this LPS and its ability to interact with different TLR molecules.