Previous studies of the systemic antibody response in necrotizing ulcerative gingivitis (NUG) have elicited varying results. The purpose of this study was to determine the humoral response to site-specific isolates of microbiota associated with NUG. Sera from 21 active NUG subjects and 21 age-sex-race matched controls were assessed for IgG and IgM antibodies to 4 clinical isolates of Prevotella intermedia and 3 clinical isolates of Treponema species. P. intermedia and Treponema strains were isolated from active and inactive sites of NUG patients and gingivitis sites of controls. P. intermedia was also isolated from noninflamed sites of the controls. IgG and IgM serum levels to these 7 bacteria were measured by ELISA. Compared to control subjects, the NUG sera exhibited significantly lower IgG and IgM levels to all 4 isolates of P. intermedia (P < or = 0.001). It was also noted that sera from NUG subjects had elevated IgM levels to all 3 spirochete isolates but significantly higher only to the spirochete isolated from a gingivitis site of a control subject (P < or = 0.005). The data suggest that failure to mount a substantial antibody response to P. intermedia may be associated with onset of disease activity in NUG. However, the assumed lack of "biological significance" between differences in antibody responses measured indicates the relationship is weak or nonexistent. It also appears that antibody response to spirochetes is not associated with onset of NUG.