BALB/c mice were immunized with crude cell surface proteins of Listeria monocytogenes V7. Approximately 1,680 hybridomas were generated after two fusions, and the monoclone C11E9 was selected and used for further characterization. The monoclonal antibody (MAb) produced by C11E9 was immunoglobulin subclass G2b with kappa light chains. Dot and colony blot results indicated that MAb C11E9 was reactive to all the L. monocytogenes (34 of 34) and Listeria innocua (6 of 6) isolates without any cross-reaction to other organisms tested. Western blot (immunoblot) analysis of crude cell surface proteins in native polyacrylamide gel electrophoresis (PAGE) indicated that MAb C11E9 reacts with a single band in each species, with a molecular mass of 174 kDa for L. monocytogenes and 182 kDa for L. innocua. The MAb reacted with one major protein band in Western blot from acid-urea PAGE for both L. monocytogenes and L. innocua. Isoelectric focusing results indicated two immunoreactive protein bands with pIs of 8.1 and 7.4 for L. monocytogenes. Sodium dodecyl sulfate (SDS)-PAGE and Western blot analysis indicated several proteins with molecular masses of 76, 66, 56, and 52 kDa for L. monocytogenes and 66, 56, and 52 kDa for L. innocua. Reaction of MAb C11E9 to washed live cells indicated the possible binding of antibody to cell surface antigen. These cell surface antigens could be removed by 1 N HCl plus 9 M urea, 2% SDS-0.5% beta-mercaptoethanol, or 4 M guanidine-HCl. The epitope of MAb C11E9 binding site was shown to be protein in nature. Periodic acid-Schiff staining and glycoprotein immunoassay indicated that carbohydrate was absent in the epitope. The cellular locations of the MAb C11E9-reactive antigens were calculated to be 76 and 90% outside and 24 and 10% inside the cell membranes of L. monocytogenes and L. innocua, respectively, for 12- to 14-h cultures.