By the fusion of mouse myeloma cells (SP2/0-Ag14) and spleen cells derived from BALB/c mice immunized with the preparation of Xanthomonas campestris pv. oryzae Ks-6-6, Os-213, Yz-32 and Yz-24, we obtained 12 hybridoma cell lines secreting monoclonal antibodies. None of the McAbs cross-reacted with the other varieties of plant pathogenetic and non-pathogenetic bacteria. The McAbs could distinguish three variant serotypes of strains. Antibody titers of ascites were about 1:10(3)-1:10(6) when measured by ELISA method. The McAbs could differentiate 6 epitopes. Based on the epitopes, the 63 strains of X. campestris pv. oryzae we collected were grouped into nine groups.