The production and testing of 10 rat monoclonal antibodies specific for strains of potato virus X (PVX) are described. Monoclonal antibodies (MAbs) with a variety of specificities were selected. At one extreme, Mab MAC 58 reacted with all 33 PVX isolates tested; in contrast, MAb MAC 67 reacted strongly with only two of 33 isolates one of which was the resistance-breaking PVX-HB against which it was raised. The reactions of the PVX isolates with the classification of PVX strains based on their effects in plants carrying different resistance genes; a preliminary serological classification is suggested. It is possible to distinguish PVX-HB from all other isolates tested using two MAbs (MAC 67 and MAC 59), and these, together with MAC 58, were produced in large quantities in ascitic fluids. Together they provide a valuable set of reagents for use in routine testing.