Monoclonal anti-CD4, anti-CD8, and anti-CD18 antibodies were applied in avidin-biotin-peroxidase complex staining using a pre-embedding immunoelectron microscopy technique. Although most of the local T cells in situ were of CD4+ subtype, local CD8+ cells generally had a lower nucleus/cytoplasm ratio and contained more cell organelles than CD4+ cells. This suggests a local activation of CD8+ subpopulation, rather than activation of the numerically predominant CD4+ cells. Topographical analysis disclosed that all lymphocytes, regardless of location, were CD18+ and that most of the CD8+ cells were located subbasally and intraepithelially, whereas CD4+ cells often occurred in small clusters deeper down in the subepithelial lymphocyte-rich band. Furthermore, CD8+ cells were often in close contact with macrophages, whereas CD4+ cells were in some instances in direct contact with plasma cells. This indicates that CD4+ cells may be involved in T cell-dependent B cell-mediated immunoglobulin synthesis, whereas CD8+ cytotoxic lymphocytes and tissue macrophages may be involved in the local pathogenetic process leading to basement membrane alterations.