SummaryFour monoclonal antibodies (MCA) were obtained to the ‘Ahlum’ serotype of beet soil‐borne virus (BSBV). On ELISA plates which had been precoated with polyclonal antibodies (PCA) all four MCA detected this serotype with a higher sensitivity than alkaline phosphatase‐labelled PCA. Three of the MCA were specific for the ‘Ahlum’ serotype, but a fourth one also detected the distantly related ‘Wierthe’ serotype. Cross‐reactions with wheat soil‐borne or oat golden stripe furoviruses were not observed. One of the MCA reacted with an epitope which is exposed along the entire length of the BSBV particles, whereas two others were specific for epitopes which are exposed on one particle extremity only. Although these latter two epitopes occur apparently on the same extremity of the particles, they seem to be different, because one is found only on the particles of the ‘Ahlum’ serotype, whereas the other one is present also on the particles of the ‘Wierthe’ serotype. The fourth MCA is specific for a cryptotope which is not exposed on the intact virus particles, but presumably on some degradation product or precursor of the viral coat protein present in crude sap preparations. All four epitopes are SDS‐labile; they are not detected on denatured viral