A gene fusion plasmid was constructed by inserting the structural gene of the firefly luciferase into a vector which allows expression of a functional recombinant scFv. The fusion protein which is expressed in the periplasm of bacteria, retains dual activity and functions in bioluminescent immunoassays. Such a genetic approach offers many advantages over chemical cross-linking of antibodies to colorimetric enzymes and may be adaptable to clinical immunoassays and high through-put drug screening assays.