TWO BIOASSAYS WERE USED TO TEST PHLOEM MOBILITY OF SELECTED XENOBIOTIC COMPOUNDS: (a) excised bean leaf assay; (b) rooted bean leaf assay. Compounds assayed were N-alkylpyridiniums with systematic variation in octanol-water partition coefficients (log K(ow)), substituted benzoic acids with about the same log K(ow) value but variable acidities. Results of the assays strongly conform, quantitatively, to the predictions of the unified mathematical model. Results also indicate that the membrane permeability value of a compound, which depends directly on log K(ow) value, is the overriding factor in determining phloem mobility. When the weak acid functionality of a compound confers increased phloem mobility, it does so principally by making the log K(ow) value, and consequently the membrane permeability of the compound more optimal.