Relative avidities of antibodies to Porphyromonas (Bacteroides) gingivalis in the sera of 15 patients having adult periodontitis and 15 healthy subjects were evaluated using an ammonium thiocyanate-dissociated ELISA. Graded concentrations of ammonium thiocyanate were added to a single dilution of serum in order to dissociate low avidity antibody binding to P. gingivalis. The concentration of thiocyanate resulting in 50% reduction in binding (absorbance) was termed the ID50 for that serum. When IgG-class antibodies were examined, the ID50 of anti-P. gingivalis antibodies in the sera of patients was significantly elevated (0.96M vs 0.71M; p less than 0.01, Student's t-test). In contrast, when IgM-class antibodies were examined no significant differences in ID50 between patients and controls were found for P. gingivalis (0.54M vs 0.53M). While the ID50 values of patient antibodies were found to be elevated relative to those of healthy controls, comparison with antibodies from rabbits immunized with P. gingivalis and with ID50 values from other human studies suggests that adult humans, in general, produce very low-avidity antibodies to P. gingivalis. It is suggested that the presence of low-avidity antibodies contributes to the pathology associated with periodontal disease.