Total rheumatoid factor (RF) activity and RF isotypes were measured in the sera of 33 patients with Yersinia infection using enzyme-linked immunosorbent assay. Twenty out of 33 patients developed reactive arthritis as a postinfectious complication. Yersinia infection does not seem to stimulate formation of RFs. The serum samples were practically negative, except two consecutive samples of one patient with Yersinia enterocolitica 0:3 triggered reactive arthritis which were strongly positive for all RFs tested. Although in Yersinia triggered reactive arthritis continuous response against Yersinia is seen, especially in the form of persisting IgA response, RF do not seem to be involved.