Previous studies on turnip crinkle virus (TCV) have suggested that the two small, centrally located ORFs, conserved in all Carmoviruses, are both required for cell-to-cell movement (Hacker et al., 1992). We now demonstrate that the cell-to-cell movement of TCV is mediated by in trans complementation of the two proteins. First, both of the putative movement proteins (MPs p8 and p9) were shown to be translated in vitro from transcripts representing the 1.7-kb subgenomic RNA. Western blot analysis, using antisera prepared against GST fusion proteins of both genes, was then used to show that the p8 but not the p9 protein accumulated to detectable levels in particulate fractions of infected cells. Cell-to-cell movement of various MP mutants in Arabidopsis was evaluated by in situ hybridization of inoculated leaves. Changes in either of the two MP genes resulted in failure of the mutants to move cell-to-cell. Coat protein was found to be unnecessary for cell-to-cell movement. Complementation of cell-to-cell movement by co-inoculating p8-defective mutants with a p9-defective mutant resulted in delayed systemic infection. In contrast, efficient cell-to-cell movement was achieved when the MP mutants were inoculated into transgenic plants expressing the corresponding functional gene(s). These experiments provide further evidence that both MP genes encoded by Carmoviruses must function in trans in the same cell in order to mediate cell-to-cell movement.