Boron is required for development of root primordia on stem cuttings of mung bean seedlings. Although exogenous boron is not required until more than 48 h after stem cuttings are placed in auxin, it may be supplied at any previous time, including during growth of the seedlings. There is a progressive loss of rooting response to auxin and boron when cuttings are aged in water prior to rooting treatment. This ageing effect is not influenced by the presence of boron but is considerably reduced by application of dilute solutions of auxin, which are themselves without effect on rooting. Cuttings prepared from seedlings grown in the presence of boron initiate fewer roots in response to low concentrations of auxin than do cuttings taken from seedlings raised in the absence of boron. Despite being required for development of root primordia, boron is antagonistic to the influence of low concentrations of supplied auxin on rooting. The possibility that boron regulates endogenous auxin levels during root development is discussed.