It is proposed that the development of periodontal disease is associated with rising levels of serum and gingival crevice fluid (GCF) IgG antibodies to specific organisms, while treatment of periodontal disease is associated with a decline in specific IgG antibodies. This study examined the immune response to Bacteroides gingivalis, a suspected periodontal pathogen, in serum and GCF of patients with adult periodontitis. Three groups of subjects were studied: (1) patients with untreated adult periodontitis, (2) patients with treated adult periodontitis, and (3) patients with gingivitis (controls). An enzyme-linked immunosorbent assay was employed using whole formalinized B. gingivalis (ATCC 33277) as antigen. Results showed that the untreated adult periodontitis patients had a humoral immune response to B. gingivalis, producing significantly higher serum levels of IgG antibody to that organism than did patients with treated adult periodontitis (p less than or equal to 0.01) or gingivitis (p less than or equal to 0.005). The untreated patients also demonstrated a local immune response to B. gingivalis in that their GCF levels of IgG antibody to that organism were also significantly higher than levels in treated adult periodontitis patients (p less than or equal to 0.005) and gingivitis patients (p less than or equal to 0.001). These results are consistent with reports by other investigators. However, ratios of GCF antibody to serum antibody in the untreated adult periodontitis group were not significantly higher than ratios in the other two groups.(ABSTRACT TRUNCATED AT 250 WORDS)