Abstract: A virulent isolate of the bacterium FN(A) was lysed with trypsin. Supernatants and precipitates of the bacterial lysates were used to prepare anti-FN serum by immunizing rabbits. Four immunogenic components were isolated using SDS-PAGE/Western-blot. Immunized rabbits were challenged,and one protective antigen was screened. After antigen isolation and purification, the N-terminal amino acid sequence was determined and an immunological test and hemolysis test were performed. Results suggested that this antigen is either a hemolysin analogue or a new antigenic substance. The antigen was able to induce a protective immune response as well as generate a marked cross-immune reaction among different bacterial strains.