In a study of members of a large family with a high prevalence of early-onset periodontitis, we sampled the subgingival microflora and characterized 40 isolates from each sample. We surveyed serum samples by enzyme-linked immunosorbent assay for antibodies reacting with any of a panel of 21 periodontal bacteria. The mother and 7 of her 13 children had early-onset periodontitis. Bacteroides gingivalis was not detected in the subgingival flora of any affected or unaffected family member, and Actinobacillus actinomycetemcomitans was isolated from only one affected child. Capnocytophaga ochracea was isolated from five of seven affected children and from none of their normal siblings. We found no significant differences among the floras from family members who had rapidly progressive, juvenile, and prepubertal forms of periodontitis. Elevated levels of serum antibody reacting with one or more of the bacteria tested were found in all family members with disease, but in only one periodontally normal family member. Both children with prepubertal periodontitis had antibodies reacting with C. sputigena, a species not found in their subgingival floras, but with none of the other bacteria tested. All remaining affected family members had antibodies to one or more serotypes of A. actinomycetemcomitans, and four had antibodies reacting with additional bacteria, including C. sputigena, Eikenella corrodens, Fusobacterium nucleatum, and Haemophilus aphrophilus. Sera from patients contained antibodies specific for putative periodontal pathogens not found in their pocket flora, and conversely, putative periodontal pathogens for which no serum antibodies were found frequently comprised a large proportion (10% or more) of the pocket flora. In no case were both the bacterium and its antibody found. These observations are suggestive of sequential infection in the early-onset forms of periodontitis and of induction of protective immunity against reinfection by the same microorganism.