A 6-week-old female BALB/c mouse was immunized with a preparation of partially purified sieve tubes of periwinkle (Catharantus roseus) plants infected with the apple proliferation mycoplasma-like organism. After fusion of the immunized spleen cells with myeloma cells, the supernatant fluids of the resulting hybridoma cultures were screened for the presence of antibodies in a differential avidin-biotin enzyme-linked immunosorbent assay (ELISA) using sieve tube preparations from either healthy or infected periwinkles as antigens. One hybridoma clone, giving a negative ELISA with the sieve tube preparation from healthy plants and a positive ELISA with that infected ones, was selected. This hybridoma also gave a negative ELISA when tested against sieve tube preparations from periwinkles affected by citrus greening or Spiroplasma citri, but a positive ELISA when tested against phyllody-affected sieve-tube preparations.