Most multicellular organisms use steroids as signalling molecules for physiological and developmental regulation. Two different modes of steroid action have been described in animal systems: the well-studied gene regulation response mediated by nuclear receptors(1,2), and the rapid non-genomic responses mediated by proposed membrane-bound receptors(3,4). Plant genomes do not seem to encode members of the nuclear receptor superfamily(5). However, a transmembrane receptor kinase, brassinosteroid-insensitive1 (BRI1), has been implicated in brassinosteroid-responses(6,7). Here we show that BRI1 functions as a receptor of brassinolide, the most active brassinosteroid. The number of brassinolide-binding sites and the degree of response to brassinolide depend on the level of BRI1 protein. The brassinolide-binding activity co-immunoprecipitates with BRI1, and requires a functional BRI1 extracellular domain. Moreover, treatment of Arabidopsis seedlings with brassinolide induces autophosphorylation of BRI1, which, together with our binding studies, shows that BRI1 is a receptor kinase that transduces steroid signals across the plasma membrane.