P. fluorescens serves as an attractive expression host for the production of stable and biologically active antibody fragments. A proprietary strain of P. fluorescens has been specifi cally developed as a protein production platform to enable rapid identification of strains capable of expressing high titers of soluble, active protein, and refl ects the additional sophistication required of engineered antibody derivatives being developed today. Pfēnex Expression TechnologyTM employs a toolbox of defined P. fluorescens strains containing custom-designed combinations of genetic elements including a set of expression vectors which encode a variety of transcription and translation regulators, as well as a collection of periplasmic secretion signals. Highthroughput transformation, growth, and expression analysis at the 96-well scale was used to rapidly identify which expression strategy and host strain combinations produced high yields of soluble, active antibody derivative. A subset of strains were analyzed by LC-MS for protein quality and then scaled-up for fermentation evaluation in order to enable rapid production of large amounts of purified protein for functional assay.