Using various antigen preparations of seven Yersinia enterocolitica serotypes (according to Knapp and Thal) and homologous rabbit immune sera the author succeeded in standardization of passive hemagglutination test. The usefulness of preparations termed "supernatant" derived from heated cells at 100 degrees C and lyophilized "crude" preparations of all serotypes were assessed using blood donors' sera and sera of subjects suspected of infection with Y. enterocolitica. It has been shown that though the diagnostic value of supernatant is higher than that of crude fraction, with the former one it is not possible to differentiate hemagglutinins against particular serotypes. More detailed assessment has been limited to serotype IA, only. An equal usefulness for passive hemagglutination of preparations termed "CA free crude" and "ethanol insoluble fraction" derived from TCA extracted cell sediment has been found.