The authors compare results of the assessment of antibodies against bacterial strains of Yersinia enterocolitica and Yersinia pseudotuberculosis. The examinations were made by the haemagglutination technique. In the reaction of the antigen human and ram erythrocytes were used as carriers. The work comprised 462 examinations of patients with an assumed Yersinia aetiology of the disease and 200 healthy controls. The values obtained during parallel examinations with antigens of Yersinia enterocolitica and Yersinia pseudotuberculosis linked to both types of blood cells were processed by statistical methods. It was revealed that when human and ram erythrocytes are used, the antibody levels correlate but are not identical. In the conclusion the authors recommend to unify the methods of different laboratories to facilitate comparison of results.