Lepromin reaction was studied with immunoperoxidase techniques using monoclonal antibodies. The skin reactions were induced by injecting Mitsuda antigen into healthy adults without a family history of leprosy. Both early (48 hours) and late (3 weeks and 2 months) reactions were examined. In the late reaction, focal collections of epithelioid cells had formed, and not only T- but also B-lymphocytes were observed around the granuloma. CD1a+ cells were also confirmed to have increased in the late reaction.