The possible function of the maize transmembrane protein TM20 in hormone transport has been investigated using immunological methods and by microinjection of TM20 cRNA in Xenopus oocytes. The existence of a similar gene in rice indicates that the overall structure of the deduced protein is conserved between these two cereals. An antibody raised against a conserved motif, in a loop between two transmembrane domains, locates the protein (TM20) in differentiating provascular cells in maize embryo. The protein has a polarized distribution within the cell in the most differentiated stages of development. Xenopus laevis oocytes microinjected with TM20 appear to modify transport activities across the plasma membrane. These results are discussed in relation to other transport proteins that influence plant development.