Lipopolysaccharide (LPS) preparations from the Bacteroides species B. endodontalis, B. intermedius, B. denticola, and B. melaninogenicus and from Fusobacterium nucleatum, Haemophilus actinomycetemcomitans, Capnocytophaga gingivalis, and Eikenella corrodens directly agglutinated erythrocytes of some kinds of animals. LPSs from the Bacteroides species B. gingivalis, B. asaccharolyticus, B. corporis, and B. loescheii and from Capnocytophaga ochracea did not possess any hemagglutinating activity. Pretreatment of LPSs with lipase and phospholipase C completely eliminated the hemagglutinating activity. The hemagglutinating activity was also affected by human serum, saliva, colistin, and polymyxin B but was not affected by sugars, amino acids, EDTA, or proteolytic enzymes. The role of this hemagglutinating activity in colonization by these microorganisms of the periodontal region is discussed.