Outer membrane protein A (OmpA) is a major outer membrane protein of Escherichia coli and other Enterobacteriaeae. Although the structural features of OmpA have been well studied, its roles in the pathogenesis of various bacterial infections have not been fully elucidated. Here, we report the generation of mouse monoclonal antibody (MAb) 49.4-15, which specifically recognizes OmpA of E. coli, using immunoblot and confocal microscopic examinations. MAb 49.4-15 might be a useful tool for studying the expression and function of E. coli OmpA.