The pathogenicity for rat ofM. arthritidisstrain Jasmin and strain 20‐P from rheumatoid synovial tissue were studied. Only strain Jasmin produced signs of illness in four of the 12 rats infected. A mycoplasma was isolated from three tissue specimens. Ten of the 48 rats investigated showed mycoplasma antibodies. Five rats showed minor inflammatory changes in the distal joints of the limbs. The failure to induce arthritic symptoms in rats inoculated with strain 20‐P from rheumatoid arthritis may be due to its 10‐year long storage outside the animal