Bi-functional antibodies with the ability to bind two unrelated epitopes have remarkable potential in diagnostic and bio-sensing applications. In the present study, bispecific antibodies that recognize human red blood cell (RBC) and the food borne pathogen Listeria monocytogenes (L. monocytogenes) were engineered. The procedure involves initial reduction of a mixture of anti-RBC and anti-Listeria antibodies followed by gradual re-oxidation of the reduced disulphides. This facilitates association of the separated antibody chains and formation of hybrid immunoglobulins with affinity for the L. monocytogenes and human RBC. The bispecific antibodies caused the agglutination of the RBCs only in the presence of L. monocytogenes cells. The agglutination process necessitated the specific presence of L. monocytogenes and the red colored clumps formed were readily visible with naked eyes. The RBC agglutination assay described here provides a remarkably simple approach for the rapid and highly specific screening of various pathogens in their biological niches.