This chapter reviews advances in the study of plant disease brought about by using monoclonal antibodies raised against fungi. It also reviews the use of monoclonal antibodies in plant disease research. Successful detection and quantitation of fungal plant pathogens in seed, plant tissue, and soil has been achieved using polyclonal antibodies in enzyme-linked immunosorbent assay and radioimmunoassays. The more traditional methods used for diagnosis of diseases caused by plant-pathogenic fungi are time consuming and require mycological expertise as well as culturing and microscopy facilities. Serological studies of plant pathogens have revealed that pathogens sometimes share antigenic determinants with their hosts. J. E. DeVay et al. proposed the idea that compatibility may be engendered by the presence of common antigens in the host and pathogen. All fungal materials should be cooled and kept at 0 to 4°C during manipulation, to slow denaturation and/or enzymatic hydrolysis of antigenic molecules.