In order to investigate arthritis-triggering, serologically cross-reactive antigens, sera from patients infected with arthritis-associated microbes, Salmonellae, Chlamydia trachomatis, and Sindbis-like alphavirus, were reacted against SDS-PAGE separated and immunoblotted Yersinia enterocolitica 0:3 antigens. These sera reacted with Yersinia to the same extent as did the control sera taken from patients with streptococcal, staphylococcal and Bordetella pertussis infections or from healthy blood donors. Moreover, the various sera produced different reactivity patterns, directed against several different antigens. Although sera from test subjects, as well as from controls including healthy individuals, recognized some Yersinia antigens, these patterns differed markedly from those recognized by sera taken from patients with Yersinia infection. Significantly, analysis of the reactivities against the different molecular weight antigen components of Yersinia revealed no dominant band or pattern which could thus have been defined as arthritis-associated.