Lymphocytes in formalin-fixed skin biopsies from patients with cutaneous acute graft-versus-host disease (aGVHD) were studied with HECA-452 (an antibody recognizing lymphocytes with skin-homing properties) and a panel of antibodies recognizing pan-B (L26 [CD20]), pan-T (L60 [CD43] and A6 [CD45RA]), and T-helper subset (OPD4) antigens in paraffin sections. Biopsies from patients with erythema multiforme (EM) were similarly studied for comparison. In both conditions, T lymphocytes stained by OPD4 were predominantly confined to the dermis, whereas those stained by HECA-452 were concentrated in the epidermis; however, there was considerable variation between cases, and overlap between findings in the dermis and epidermis. Lymphocytes similarly studied in paraffin sections of liver, salivary gland, and gut affected by aGVHD were essentially unreactive with HECA-452, although they were largely stained by pan-T markers and showed some comparable reactivity with OPD4. The findings suggest that aGVHD of the skin is mediated by a different set of lymphocytes than in gut organs, and may have a similar immunologic mechanism to EM.