The in planta localization of a full-size immunoglobulin G (IgG) antibody and of its derived Fab fragment were determined using immunogold electron microscopy on transgenic Arabidopsis thaliana plants. Although it is widely assumed that antibodies, following assembly in the lumen of the endoplasmic reticulum, migrate via the secretory pathway to the cell surface, until now their subcellular in planta localization has not been demonstrated. Here, we show that both IgG and Fab fragment accumulate within intercellular spaces of leaf mesophyll cells of Arabidopsis. Using immunoblot analysis on leaf intercellular fluid, we demonstrate that these microscopically detected proteins are integral and functional antibodies or Fab fragments. The fact that a full-size IgG is not physically restricted from passage through the cell wall, despite having a molecular mass of 146 kD, questions the generally accepted limitations in cell wall porosity. These results are of particular significance in any plantibody approach that intends to modulate extracellular antigen function.