Phenotypic expression of resistance to potato Y potyvirus (PVY) and potato A potyvirus (PVA) was tested in 24 potato cultivars and an advanced breeding line using graft-inoculation under controlled conditions in the glasshouse. Resistance phenotypes were determined based on symptom expression and systemic infection detected with DAS-ELISA. Tubers were harvested from the PVA-inoculated plants and tested for PVA with ELISA. Sixteen potato cultivars expressed hypersensitive resistance (HR) to the strain group YO of PVY. Ute expressed extreme resistance (ER) to PVY (strain groups YO and YN) and PVA, and eight cultivars (Amazone, Bintje, Fambo, Posmo, Record, Rosamunda, Saturna and Van Gogh) expressed ER to PVA. These cultivars produced no PVA-infected tubers (tubers of Record were not tested). Matilda and Nicola expressed HR to PVA. The tubers of graft-inoculated Matilda produced no PVA-infected shoots, whereas shoots from Nicola tubers developed necrosis and severe mosaic symptoms and were PVA-infected based on results from ELISA. Comparison with purified PVA antigen (using ELISA) indicated that the secondarily infected shoots of the 14 PVA-susceptible cultivars contained 206-804 ng of PVA antigen per gram of leaf tissue.