To test the hypothesis that auxin-binding protein 1 (ABP1) is a receptor controlling auxin-mediated plant cell expansion, ABP1 complementary DNAs were expressed in a controllable fashion in tobacco plants and constitutively in maize cell lines. Induction of Arabidopsis ABP1 expression in tobacco leaf strips resulted in an increased capacity for auxin-mediated cell expansion, whereas induction of ABP1 in intact plants resulted in leaves with a normal morphology, but larger cells. Similarly, constitutive expression of maize ABP1 in maize cell lines conferred on them the capacity to respond to auxin by increasing cell size. These results support a role of ABP1 as an auxin receptor controlling plant growth.