Monoclonal antibodies (MAbs) were produced toRhizomonas suberifaciensstrain CA1. Initial screening of MAbs was performed with strains CA1 and FL1 ofR. suberifaciensand 82 strains of other genera in antibody capture indirect enzyme‐linked immunosorbent assay (ELISA) tests. One MAb (MAb‐Rs 1) reacted with both strains ofR. suberifaciensand another (MAb‐Rs 2) only withR. suberifaciensstrain CA1. Both MAbs were further screened for cross‐reactivity with 28 strains of R.suberifaciens,four strains ofRhizomonassp., and 56 strains of unknown bacteria isolated from lettuce, prickly sowthistle and melon roots with symptoms of corky root. All these strains were tested for pathogenicity on lettuce, DNA homology with DNA ofR. suberifaciensstrain CA1, and cross‐reactivity with both MAbs in indirect antibody capture and indirect double antibody sandwich (DAS) ELISA tests. All strains ofR. suberifaciensreacted with MAb‐Rs1 in both indirect ELISA tests, while none of the other strains reacted. In antibody capture indirect ELISA tests, MAb‐Rs2 had affinity to strains ofR. suberifaciensfrom California and New York but not to those from Florida, Wisconsin and one from New York. In indirect DAS ELISA tests, MAb‐Rs2 reacted with all strains of R.suberifaciens.The indirect DAS ELISA tests were more sensitive than the indirect antibody capture tests.