Tissue blot immunoassay has been successfully employed in investigations of many plant viruses and to a lesser degree fungi and mycoplasma-like organisms. Detection enzymes for use in tissue blot immunoassay should be carefully evaluated before the system is established. Horseradish peroxidase is commonly employed in enzyme-linked immunoassay for detection of a number of plant pathogens. Monoclonal antibodies provide a level of standardization that is not possible with polyclonal antibodies. Construction of hybridomas secreting antibodies to plant viruses is a well-established procedure, and monoclonal antibodies have been produced to numerous plant pathogens. Tissue blot immunoassay retains advantages that many other assay methods offer, including specificity and reliability. In tissue blot immunoassay, the substrates that yield insoluble colored products precipitating at the site of enzyme reaction should be the choice. Alkaline phosphatase-antibody conjugates are generally available in plant pathology laboratories and are convenient to investigators.