Abstract: An objective of the present invention is to provide a therapeutic agent for treating rheumatoid arthritis, juvenile rheumatoid arthritis, macrophage activation syndrome, septicemia, and FR-? expressing leukemia, which induces apoptosis in activated macrophages and folate receptor beta (FR-?) expressing leukemia cells to specifically destroy these cells. An FR-? monoclonal antibody of the present invention is preferably an IgG-type monoclonal antibody which specifically reacts with a human-type FR-? antigen and is produced from a clone resulting from immunization with an FR-? expressing B300-19 cell. The FR-? monoclonal antibody of the present invention specifically reacts with the FR-? antigen of activated macrophages and FR-? expressing leukemia cells and a therapeutic agent of the present invention contains an FR-? antibody immunotoxin which causes apoptosis in activated macrophages and FR-? expressing leukemia cells, as an active ingredient.