Two reliable systems for the cell-mediated passive transfer of adjuvant arthritis have been developed. Donor rats were sensitized with Mycobacterium butyricum in mineral oil. In the first system, intravenous injection of adjuvant-sensitized donor lymph node or spleen cells into adultthymectomized, lethally irradiated, bone marrow cell-reconstituted syngeneic rats induced arthritis in the recipients. In the second system, adjuvant-sensitized donor lymph node or spleen cells were cultured in vitro with concanavalin A; these cells induced arthritis in normal recipients as well as in thymectomized, irradiated, bone marrow cell-reconstituted recipients. The passively transferred disease in both systems resembled classical adjuvant-induced arthritis clinically, radiographically, and histologically. Neither irradiated, adjuvant-sensitized donor cells nor cells from donors not injected with complete adjuvant could passively transfer arthritis.