A sporadic polyarthritis was observed in rats during induced regression of lymphosarcoma. Mycoplasma arthritidis were cultured from joint lesions and tumor tissue. An investigation was made of the respective roles of the PPLO, the tumor-necrotizing bacterial polysaccharide, and the tumor regression in production of the polyarthritic lesions. The significant conditions for the production of polyarthritis in these tumor-bearing rats are (1) the presence of Mycoplasma arthritidis and (2) the concurrent or prior necrosis of tumor tissue. A host response to the necrotic tumor appears important since rats with recently regressed tumors, but without remaining necrotic tissue, have enhanced susceptibility to arthritis when injected with PPLO.