The humoral immune response, especially the production of IgG and IgA, is considered to have a protective role in the pathogenesis of periodontal disease, but the precise mechanisms are still unknown. In order to determine local IgG and IgA production, we investigated the presence of human IgG and IgA subclass mRNA-bearing plasma cells within periodontal tissue by in situ hybridization using digoxigenin-labelled oligonucleotide probes in 24 gingival biopsy samples (pocket depth > or = 5 mm) which were obtained from eight patients with adult periodontitis. Furthermore, we examined IgG and IgA subclass proteins and digested IgA1 Fab portions in the gingival crevicular fluid (GCF), corresponding to the sites from which the tissues were taken, by ELISA. IgG and IgA subclass mRNA-expressing cells were detected in all serial formalin-fixed/paraffin-embedded gingival tissue sections sampled. Plasma cells showed strong cytoplasmic staining with a high contrast and a good retention of morphology with these probes. IgG1 mRNA-expressing cells were predominant (mean 63%) and IgG2 mRNA-expressing cells were present at around 23% of total IgG plasma cells, while IgG3 and IgG4 mRNA-expressing cells were present to a much lesser extent (3% and 10%, respectively). Similar proportions of IgG subclass proteins in GCF were detected, which were also consistent with 'normal' serum levels. In terms of IgA subclass, IgA1 mRNA-positive cells were predominant (mean 65.1%, P < 0.001). In contrast, IgA2 protein in the GCF samples were detected at higher concentrations than IgA1 (P < 0.001). The ratio of total IgG to IgA mRNA-positive plasma cells was approximately 7.5:1. There was a good correlation between the amounts of IgG subclass proteins in GCF and the number of IgG subclass mRNA-positive cells in the same sites, but not between IgA subclass proteins and the number of IgA subclass mRNA-positive cells. These data suggest that IgG and IgA subclass proteins can be locally produced in the periodontitis gingiva. In addition, as we detected IgA1 Fab fragments in GCF, this is further confirmation that secreted IgA1 protein in GCF may be digested by periodontal bacteria.