Visual inspection of seed potato plants is a routine method in virus diagnosis prior to enzyme-linked immunosorbent assay (ELISA) and polymerase chain reaction (PCR) procedures. In this study, the visual rating of potato plants for virus symptoms, together with both these laboratory procedures, was subject to an accreditation by International Standard ISO/IEC 17025 in the plant protection office of Lower Saxony, Germany. The validation of the rating was applied to two samples with 100 single potato plants in each. Each sample was investigated using four ratings, each carried out by three inspectors, consecutively. Standardization was established using inspection under available daylight to assess the degree of virus symptoms. An inspection procedure is suggested which (i) takes into account the light conditions during the inspection and (ii) uses a calibration of the ratings given by the three inspectors obtained from prior inspection jointly of another sample. For each rating the number of plants showing virus symptoms was recorded and ratings were compared using standard statistical methods (mean value, standard deviation and coefficient of variation).