Mononuclear cell populations were studied in gingival biopsy specimens from adolescents (n = 10) with at least one periodontal pocket with an increased probing depth (greater than 5 mm). The marginal bone loss was measured on radiographs, subgingival plaque samples were collected from the lesions, and the microbial flora was identified. Specimens from gingivitis lesions (n = 5) were used as controls. The mononuclear cell populations in the specimens were detected by using monoclonal antibodies defining functional T-lymphocyte subpopulations, B lymphocytes, and monocytes. All gingival specimens from patients with increased probing depth showed large lymphocyte infiltrates, most of which were CD 3-antigen-positive cells (T lymphocytes). Few (2%) infiltrating T cells expressed receptors for interleukin-2. B cells were detected in most specimens from the periodontitis group and varied from less than 1% to 21%. This study indicates that lesions in adolescents with early signs of periodontitis are characterized predominantly by T-cell lesions with relatively few cell aggregates of B cells present.