Phylogenetic investigations led to the definition of a new group of bacteria called haemotrophic mycoplasmas (haemoplasmas).The life cycle of said globally spread bacteria is dependent on their intimate contact with their host erythrocytes. Illnesses showing symptoms of a haemolytic anaemia are usually found in cats and swine. Haemoplasmas are small pleomorphic bacteria (0.3-3 microm) with a very small genome (745-1245 kbp).To date there is very limited knowledge on their biology and the host-bacteria-interactions (immune response, pathogenesis) since these bacteria are yet not culturable. Applying modern molecular biological techniques we succeeded during the last few years in gaining new facts on the antigen structure of M.suis as well as on the immunology and pathogenesis of the porcine eperythrozoonosis.Thus we detected three main antigens two of which we expressed recombinant in our laboratory. These surface-exposed antigens serve as a basis for establishing serologic assays, and are vaccine candidates, too. Preliminary studies allowed us to find the function of an adhesin of M. suis for one of the two proteins.