Changes in the Langerhans' cells (LC) in seven cases of leprosy were observed using the monoclonal antibody OKT6. In the epidermis of the lesions of most of the leprosy cases, the processes of LC were reduced or diminished. Some LC were disintegrated. In two BT cases in the upgrading (type 1) reactional stage, either increased numbers and aggregation of OKT6-positive cells in clusters or their disintegration was seen in different areas of the epidermis.