Transformed plants expressing the 3' two-thirds of the cowpea chlorotic mottle virus (CCMV) capsid gene were inoculated with a CCMV deletion mutant lacking the 3' one-third of the capsid gene. Although the deletion inoculum replicates in inoculated cells, systemic infections occur only if recombination restores a functional capsid gene. Four of 125 inoculated transgenic plants, representing three different transgenic lines, became systemically infected. Analysis of viral RNA confirmed that RNA recombination had united the transgenic messenger RNA and the challenging virus through aberrant homologous recombination.