Ten stable hybridoma cell lines, MAsl-10, secreting monoclonal antibodies specific to the causal bacterium of pear leaf scorch (PLS), Xylella fastidiosa, were produced. The monoclonal antibodies can detect 3 x 10(5) PLS-bacterium cells by indirect enzyme-linked immunosorbent assay (ELISA). In the antibody titer determination by indirect ELISA, hybridoma-culture supernatant from clone MA4 had the highest titer of 20480. In the antibody specificity tests, nine of the 10 monoclonal antibodies did not crossreact with 14 other bacterial strains belonging to nine genera. Only the antibody from hybridoma clone MA1 cross-reacted with Xanthomonas campestris pv. campestris and X. campestris pv. vesicatoria. In western blot analysis, all the monoclonal antibodies recognized the major 46.9-kDa polypeptide from all 12 X, fastidiosa strains and a distinct 21.5-kDa polypeptide only from PLS bacterium. In tissue-blotting detection, the PLS bacteria were specifically detected in blots of tissue sections from infected pear with the antibodies developed.