A cross-reactivity among some strains of Coxiella burnetii and chlamydiae with immune rabbit and mouse sera in ELISA and immunoblot analysis was observed. In the latter, the cross-reactivity disappeared after a treatment of C. burnetii or C. psittaci with proteinase K, which indicates that only proteins were involved. The observed cross-reactivity was not influenced by host chick embryo yolk sac proteins. After adsorption of immune rabbit sera with homologous corpuscular antigens the cross-reactivity disappeared. The possibility of influence of such cross-reactivity on serological diagnosis of C. burnetii or chlamydiae infections is discussed.