Human IgM, IgG and IgA responses after infection with Yersinia enterocolitica serovar O3 were studied by immunoblotting sera against whole-cell homogenates of a plasmid-containing strain of Y. enterocolitica O3 and a plasmid-free strain derived from it; each strain was grown in conditions expressive for the plasmid. The antibodies observed were directed against several plasmid-encoded polypeptides. The response against different bacterial components decreased uniformly with time and the persisting antibody production was directed against several epitopes. Strong reactions to the prominent plasmid-specified antigens of mol. wts (10(3] 26, 34, 45 and 52.5 were found more often with IgG-class antibodies than with IgM or IgA; the latter immunoglobulins recognised, respectively, antigens of mol. wt (10(3] 26 and 45 (IgM) and 26, 34 and 52.5 (IgA). Immunoblotting of sera from patients with yersinia-triggered reactive arthritis did not reveal any antigens that were involved additionally or specifically. However, IgA-mediated recognition of certain antigens of mol. wts (10(3] 26, 34 and 52.5 tended to persist longer in the arthritic patients.