Yersinia enterocolitica O:3 was maintained in primary cultures of human synovial cells for 6 weeks as cultivable organisms and thereafter for 2 more weeks as antigen aggregates containing specific lipopolysaccharides (LPS). Some seemingly intact bacteria were "ghosts," bacterial rods possessing LPS but not DNA. The prolonged persistence of yersiniae, and consequently of Yersinia antigens, in synovial cells may be the cause of the maintenance of the inflammatory host responses in the joints of patients with reactive arthritis due to Yersinia infection.