Mycoplasma hyopneumoniae causes swine enzootic pneumonia (SEP) and leads to economic loss worldwide. The mechanism of pathogenesis is still not clear. Since this pathogen remains extracellulary after infection, the surface proteins on M. hyopneumoniae should play very important roles in adhering and affecting tracheal mucosal cells. Therefore, the potential of using the surface proteins as the basic to develop molecular vaccine is currently being investigated. The recombinant clone expressing the 42 kDa protein was isolated from the