Serpulina hyodysenteriae vaccine

According to the present invention a vaccine can be prepared containing a mutant Serpulina hyodysenteriae which is defective in its production of biologically active hemolysin. The mutation by which Serpulina hyodysenteriae is made defective in its production of hemolytically active hemolysin is established by means of genetical engineering techniques. Such mutations comprise e.g. deletion of part or the entire gene coding for hemolysin and/or nucleotide sequences controling the production of hemolysin, or insertion of an extra nucleotide or polynucleotide into the gene encoding hemolysin and/or the nucleotide sequences controling the production of hemolysin, or a combination of said deletion and insertion. These vaccines are useful in the prevention of Serpulina infections in susceptible animals such as swine.