A satellite RNA (T73-satRNA) gave reduced symptom severity on tomato plants when coinoculated with an ordinary strain of cucumber mosaic virus (CMVO). cDNA for T73-satRNA was introduced into a binary vector (pTOK162) through a homologous recombination in an Agrobacterium tumefaciens cell, and then transferred to leaf disks of tomato. Stable integration and transcription of the cDNA in the regenerants were verified by Southern and northern blot hybridizations, respectively. Upon inoculation with CMV-O, the transformants exhibited very slight symptoms of CMV, grew normally, and finally set fruits, whereas untransformed wildtype tomato plants showed very severe symptoms, and their growth was retarded and formed few fruits. Agarose gel electrophoresis of total RNA from CMV-O-inoculated transformants detected RNA molecules corresponding to T73-satRNA.