Tobacco plants transformed with a gene encoding a truncated cucumber mosaic virus (CMV) 2a replicase protein are resistant to systemic CMV disease. Experiments using protoplasts derived from plants of two R2-generation CMV-resistant transgenic plant lines (lines R2-2 and R2-5) showed that resistance operates at the single cell level. Low levels of CMV-specific RNAs were detected in CMV-inoculated protoplasts obtained from both R2-2 and R2-5 plants indicating that resistance is due at least in large part to a marked but incomplete suppression of virus replication. Leaves of immature plants belonging to line R2-2 occasionally exhibited local chlorosis when inoculated with high concentrations of CMV. Areas of local chlorosis were sites of low but detectable levels of CMV RNA, CMV virions, and CMV replicase activity, but did not act as foci for subsequent systemic disease. An antiserum raised against the CMV 2a replicase protein overexpressed in Escherichia coli was used to detect the presence of trace amounts of the truncated CMV 2a replicase protein in CMV-resistant transgenic tobacco plants. It was concluded that expression of the transgene, potentially as protein, engenders resistance primarily by suppressing virus replication but may also, to a lesser extent, do so by inhibiting systemic movement of the virus.