In this paper, the immunological cross-reaction between Yersinia enterocolitica 0:9 (Y-9) and Brucella abortus has been studied with the following results: It has been demonstrated that serological cross-reaction exists between the organisms and the agglutination titer of Y-9 antigen and anti-Brucella human sera is lower than that between Brucella antigen and Yersinia anti-sera. Y-9 antigen can only be agglutinated by Brucella monospecific serum "A", but not by serum "M" or "R". 24-48 hours after the injection of Y-9 antigen either in the form of whole cells or purified protein, a clear skin cross-reaction was observed in a guinea pig sensitized with Brucella 104M. But the skin cross-reaction of a guinea pig sensitized with Y-9 to Brucella antigen was rather weak. In case the antigen used in skin test is similar to that used for sensitization, similar results are obtained at 24 and 48 hours for Brucella antigen, but for Y-9, the reaction at 24 hours is stronger than that at 48 hours. In the protection test, the protection rate against B. melitensis M13 virulent strain in guinea pigs immunized with Y-9 and B. abortus are 33.3% and 90% respectively, while those against B. abortus 104M in C57BL inbred mice immunized with Brucella antigen and Y-9 are 90% and 50% respectively.