Type 1 (reversal) reactions are the most common immunological complications of leprosy. These episodes of delayed hypersensitivity produce severe local immunopathology and ultimately nerve damage. To date, the Mycobacterium leprae antigens associated with type 1 reactions have not been identified. Using monoclonal antibodies to defined protein and carbohydrate M. leprae epitopes (65, 35 and 28 kd and lipoarabinomannan [LAM]) in a two-step immunoperoxidase staining technique, M. leprae antigens were demonstrated in skin and nerve biopsies from patients in reversal reaction. Antigen presence and staining patterns were similar in skin and nerve lesions, implying that the pathological processes are similar in the two sites. Antigens were present both in macrophages and Schwann cells but also as a diffuse extracellular infiltrate associated with the inflammatory infiltrate. The 28-kd antigen was present most strongly and may be a potential candidate antigen for initiating type 1 reactions. LAM also stained strongly and persisted after treatment. The possible roles of LAM and 65 kd in the cellular events of type 1 reactions are discussed.