Transgenic tomato plants (Lycopersicon esculentum Mill.) expressing cucumber mosaic virus (CMV) satellite RNA fused to a gene for β-glucuronidase were produced using Agrobacterium-mediated transformation. The R1 progeny of self-crossed R0 plants were challenge-inoculated with virion or RNA preparations of CMV or tomato aspermy virus (TAV). The transgenic plants challenged with CMV-1 showed mild disease symptoms in the first 2 weeks postchallenge followed by a decrease in symptoms, resulting in little difference between the transgenic and uninfected control group by the fourth week. Enzyme-linked immunosorbent assay results showed about a 10-fold decrease in virus accumulation in the transgenic plants compared to controls. Tolerance was evident only in plants that contained the recombinant insert and produced mature unit-length satellite RNA after CMV infection. Plants challenged with TAV showed no significant tolerance to virus-induced symptoms.