Previous studies have been performed for the sero-identification of selected species of Bacteroides by immunofluorescence antibody techniques and enzyme-linked immunosorbent assay using species-specific rabbit antisera to B. gingivalis, B. intermedius, and B. melaninogenicus. However, these studies have not commented on the serological cross-reactivity between these 3 species of black- pigmented Bacteroides. For the cross-reactivity study, antisera to B. gingivalis ATCC 33277, B. intermedius ATC C25261 and B. asaccharolyticus ATCC 25260 were raised from rabbits. Preliminary study for observing the cross-reactivity between these species was performed by indirect immunoflourescence technique. Immunoabsorption of the antisera was done with bacterial cells from the other species and the species-specificity of the antisera was conformed by the absence of reactivity with bacterial strains from the other species by indirect immunofluorescence technique and enzyme-linked immunosorbent assay. Three representative unabsorbed antisera cross-reacted strongly with cells from the other species. Especially, anti-B. asaccharolyticus ATCC 25260 antiserum showed a strong cross-reactivity with B. gingivalis ATCC33277. After immunoabsorption of 3 representative antisera with the other species, the cross-reactivity was found only between B. gingivalis ATCC 33277 and B. asaccharolyticus ATCC 25260. Further study would be necessary to clarify the cross-reactivity between important oral black-pigmented Bacteroides from subgingival plaque or bacterial colonies for rapid identification.