Monospecific antibodies were successfully obtained by using unique protein bands of the quarantine bacterium Xylella fastidiosa, the causal agent of Pierce's disease of grapevine, and by applying gel electrophoresis and electroblotting. The monospecific antibodies directed against the two best suited protein bands (20.7 kD, 19.8 kD) did not show any cross-reaction on a Western blot, a solid phase ELISA (Enzyme-Linked Immunosorbent Assay) or an IFAS (Indirect Fluorescent Antibody Staining). A monospecific antiserum was generated in rabbits directed against the 20.7 kD and 19.8 kD protein bands, and was applied in an ELISA. Formerly cross-reacting bacteria isolated from grapevine cuttings did not show any antibody reaction with this antiserum. When grapevine cuttings were pre-infiltrated with a suspension of 4.15 x 10(7) bacteria the detection limit was found to be 10(6) - 10(5) cells in the sap extract using ELISA. Titer used in all assays of the monospecific antiserum was 1:100. Using pure cultures of Xylella fastidiosa the detection limit of the monospecific antiserum in an indirect ELISA was 8.30 x 10(5) cells/ml.