Rats of five different strains were injected intravenously with live Yersinia enterocolitica O:8 or Yersinia pseudotuberculosis III. Twelve to 18 days after inoculation of Y. enterocolitica O:8, arthritic symptoms appeared in the hindpaws of SHR rats. They started with erythema, followed by swelling and painful movement, closely resembling findings in adjuvant arthritis. Histologically, the inflammation was dominated by neutrophils in two cases, whereas in two others the inflammatory cells were predominantly lymphocytes. Two of five rats yielded positive bacterial cultures from the joints. Altogether, arthritis was observed in five of nine SHR surviving beyond day 14 after the bacterial inoculation. Rats of Lewis, Wistar, WKY and Zucker strains did not develop any signs of arthritis after intravenous injection of Y. enterocolitica or pseudotuberculosis. Also in the SHR rats, Y. pseudotuberculosis failed to induce arthritis.