Solutions of 14C-labeIled amino acids were allowed to percolate through the xylem vessels of tomato stem parts. The amounts absorbed by the stem tissues were determined by analyses of the collected fluid. The protein amino-acids glycine and L-a-alanine were strongly retained by the stem tissues and partly metabolized. Hardly or no absorption, however, was found for either the protein amino-acids L-proline and L-glutamic acid, or for the “unnatural” amino-acids D-a-alanine, D-glutamic acid and a-aminoisobutyric acid. It is concluded that during xylem translocation of amino-acids a selective uptake may occur by binding sites from which at least some have a stereo-specific affinity for these compounds.