The gene encoding the 25 kDa major outer membrane protein (MOMP) of Legionella pneumophila was transformed into Escherichia coli JM 83 and the resultant E. coli LP 116 clone expressed the Legionella-MOMP. Compared with the parent E. coli strain, the clone showed a fivefold increase in opsonin-independent binding to U-937 cells. Furthermore, this gene was incorporated by electroporation into a low virulence derivative of Leg. pneumophila which showed reduced expression of the MOMP but enhanced expression of a 31 kDa protein in the OMP profile. After electroporation, the attenuated strain showed an increased expression of the MOMP while the 31 kDa protein was eliminated and virulence for the chick embryo was re-established. The use of a monoclonal antibody specific for the MOMP abolished virulence and adherence. These studies suggest that the 25 kDa MOMP of Leg. pneumophila serves as an adhesive molecule for host cells and that this protein plays a major role in the virulence of the organism for the chick embryo.