In cooperation with the Department of Molecular Biology of the Agricultural University Wageningen, cDNA-probes were produced for potato spindle tuber viroid (PSTV) and potato virus YN (PVYn) for use in molecular-hybridization tests. The probe for PSTV contains a full-length copy of the viroid-RNA: the probe for PVYN contains a copy with a length of about 2500 base pairs, which is 25% of the total lenght of the PVYn-RNA. The probes were labelled with 32P by ”nicktranslation”. Nitrocellulose filters were used as solid support. 3 pi samples were spotted onto dry filters that were pretreated with 20 x Standard Saline Citrate. The molecular-hybridization test is a quick method – results can be obtained within 4-5 days – and only 3 µl of sample is needed. Moreover the method is cheap in comparison to the tomato-PAGE method.