To study the first stages of interactions between bacteria and mammalian host cells, it is necessary to locate the surface antigens involved in cytodhesion processes. The identification of a bacterial adhesin is normally based on experimental evidence of adherence inhibition in vitro by the purified protien or, more often, a monoclonal antibody (MAb) directed against this protein, as well as visual data from immune electron microscopy. Alternatively, the adhesive function can be demonstrated via the construction or selection of mutants lacking the protein in question. The considerable experimental effort required to identify a single adhesion factor is one of the reasons why only a few such proteins have been found for most bacterial species so far, although in view of the complex nature of the attachment process, a larger number of adhesins can be expected to be involved.