Unique functions of plant pathologists include diagnosis of plant diseases, detection and identification of disease-causing organisms, and research on biotic pathogens and host-pathogen interactions. This chapter aims to review for plant pathologists and nonplant pathologists alike the status on the production and application of monoclonal antibodies (mAbs) in the field of phytobacteriology. It focuses on the utilization of mAbs for the detection, identification, characterization, and analysis of variants and strains of plant-pathogenic bacteria, spiroplasmas, and mycoplasma-like organisms. Hybridization of antibody-forming lymphocyte cells with malignant myeloma cells results in hybridomas which combine the parental traits of specific antibody secretion and continuous growth. Serum-free media that support the growth of several parent myeloma lines have been described. These media are usually mixtures of Rosewell Park Memorial Institute and Dulbecco's modified Eagle's medium supplemented with hormones, transferrin, lipids, trace elements, and other factors. Feeder cells or conditioned medium of some sort increase the ability of cultured cells to grow at very low densities.