Monoclonal IgM-antibodies specific for arthritogenic erysipelas bacteria (Erysipelothrix rhusiopathiae, serovar 2, strain T28) were isolated from rats suffering from erysipelas polyarthritis. Four of them (C52, D9, E51, R117) were administered to syngeneic Lewis rats. While D9 and an unrelated rat-IgM-antibody showed no effect, C52 protected partially and R117 as well as E51 fully from all symptoms of erysipelas polyarthritis. Prevention of the disease was associated with a lack of antibody-formation against the erysipelas bacteria. There is evidence that prevention is not due to classical passive immunization, but rather to induction of host mechanisms efficiently activated by "inductive" monoclonal antibodies.