Studies were initiated to investigate the antigenic relationship among oral species ofBacteroides. Antigenic differences among theBacteroideswere assessed with an ELISA using antisera from immunized rabbits and sera obtained from subjects with periodontal disease. Multiple species of Bacteroides were examined, including:B. gingivalis, 2 homology groups ofB. intermedius, B. melaninogenicus, B. denticola, B. loescheii, B. oralis, B. levii, B. gracilis, B. buccae.andB. corporis.Both rabbit and human sera reacted with distinct surface antigenic determinants in each of the species of Bacteroides. Additional relationships were defined, such that the 2 homology groups ofB. intermediusexhibited a similarity of approximately 30–40%, while theB. melaninogenicusandB. loescheiishowed a 20–25% similarity of surface antigens.B. denticolawas antigenically related to theB. intermediusand B. melaninogenicus strains by 10–15%. The remaining cross‐reactivities were generally less than 5% among theBacteroidesspecies. The findings are consistent with specific surface antigenic determinants on the oral Bacteroides spp. and suggest the ability to detect specific responses to these organisms in periodontal disease p