The development of a monoclonal antibody directed against HLA DR (Ia-like) antigens of B cells and monocytes but not against normal peripheral human T cells suggested that this antibody might be used as a marker of B cells and monocytes in tissue sections. The T cell nature of large plaque (atrophic) parapsoriasis has recently been demonstrated by the immunoperoxidase technic. Immunoperoxidase examination of serial sections of tissues from two cases of large plaque parapsoriasis with one T cell antiserum, two monoclonal T cell antibodies, and one monoclonal reagent directed against HLA DR indicated that T cells in the cutaneous infiltrates were also HLA DR-positive. Evidence is accumulating that HLA DR positivity may be expressed by activated T cells. The findings here therefore suggest that many of the T lymphoid cells in two cases of large plaque (atrophic) parapsoriasis examined were activated in nature, and that HLA DR may not be a specific marker for B cells and monocytes in certain pathologic conditions. Caution should therefore presently be exercised in attempting to use this marker for the specific identification of B cells and monocytes in pathologic specimens, without simultaneous testing for T cell markers.