The presence of mononuclear cells was studied in gingival biopsies from seven children exhibiting phenytoin(PHT)-induced gingival overgrowth, three children with gingivitis and a control group consisting of three children without clinical signs of inflammation. The mononuclear cells were detected using monoclonal antibodies defining functional T-lymphocyte subpopulations, B-lymphocytes and monocytes. Gingival biopsies from the individuals in the PHT-group showed a substantial number of mononuclear cells. The distribution of mononuclear cells in separate individuals were as follows: 69-95% OKT3 +/Leu4+ cells (T-lymphocytes), 50-64% OKT4 +/Leu3+ cells (T-helper phenotype) and 29-46% OKT8+ cells (T-suppressor/cytotoxic phenotype). None of the biopsies in the PHT-group contained more than a few scattered plasma cells. The vast majority of all mononuclear cells present in the biopsies reacted with OKIa1, a monoclonal antibody defining the HLA-DR framework. In contrast, biopsies from the control group and the gingivitis group contained few mononuclear cells, the majority of which were T-cells. This suggests that immunologic reactions mediated by T-cells may play a role in the pathogenesis of the PHT-induced lesion.