Fifty-eight scalp biopsies were immunohistologically investigated with monoclonal antibodies against HLA-ABC, HLA-DR, and T6 antigens. The following 3 groups were compared: control biopsies obtained from healthy volunteers (n = 5) or patients with unrelated scalp diseases (n = 6); biopsies from untreated alopecia areata (AA), obtained either from untreated patients (n = 19) or from the untreated side in patients receiving unilateral treatment with the contact allergen diphencyprone (DCP) (n = 13); biopsies obtained from the treated side in patients receiving unilateral treatment with DCP (n = 13). While HLA-ABC antigens were strongly expressed by epidermal keratinocytes and the infundibular epithelium of hair follicles in all biopsies, these antigens were either not detectable or only faintly expressed on the subinfundibular epithelium and the hair matrix in the control series. By contrast, 30 out of 32 biopsies from untreated AA showed expression of HLA-ABC antigens on hair matrix epithelium, and the subinfundibular epithelium was HLA-ABC-positive in 15 out of 32 cases. In the biopsies from treated AA, HLA-ABC antigens were expressed on hair matrix epithelium in 9 out of 13 cases, and on the subinfundibular epithelium in 1 case. In the controls and untreated AA, HLA-DR expression was confined to dendritic cells in the epidermis and the follicular infundibulum. Its expression on hair matrix epithelium was found in 15 out of 32 biopsies from untreated AA and in 4 out of 13 biopsies from treated AA. In the control series, intrabulbar T6+ dendritic cells were either absent or present in low numbers. High numbers of intrabulbar T6+ cells were present in 7 out of 32 biopsies from untreated AA and in 0 out of 13 biopsies from treated AA. The data show that abnormal expression of class I major histocompatibility (MHC) antigens on hair matrix epithelium is a constant feature in AA, whereas class II MHC antigens are less frequently expressed. Topical immunotherapy with DCP, which induced expression of HLA-DR in epidermal keratinocytes in 6 out of 13 cases, reduced the abnormal expression of both HLA-ABC and -DR antigens in the epithelium of lower hair follicles in AA.