Abstract: An antigen which produces immunity against homologous or heterologous challenge with babesia of cattle. The antigen is immunoreactive with a monoclonal antibody or polyclonal antisera recognising a protein located on the surface of babesia-infected erythrocytes and within a spherical or mitochondrion like organelle. The antigen can be prepared by (i) preparing nucleic acids from babesia infected erythrocytes depleted of leucocytes; (ii) forming a cDNA or genomic library from nucleic acids obtained in step (i); (iii) screening said library formed in step (ii) with a suitable probe to identify clones of interest; and thus providing DNA inserts for an expression vector which may be used to transform an appropriate host; (iv) obtaining a recombinant polypeptide from said transformed hosts which is protective against babesiosis.