Both animals and plants use steroids as signalling molecules during growth and development. Animal steroids are principally recognized by members of the nuclear receptor superfamily of transcription factors. In plants, BRI1, a leucine-rich repeat (LRR) receptor kinase localized to the plasma membrane, is a critical component of a receptor complex for brassinosteroids. Here, we present the first evidence for direct binding of active brassinosteroids to BRI1 using a biotin-tagged photoaffinity castasterone (BPCS), a biosynthetic precursor of brassinolide (the most active of the brassinosteroids). Binding studies using BPCS, (3)H-labelled brassinolide and recombinant BRI1 fragments show that the minimal binding domain for brassinosteroids consists of a 70-amino acid island domain (ID) located between LRR21 and LRR22 in the extracellular domain of BRI1, together with the carboxy-terminal flanking LRR (ID-LRR22). Our results demonstrate that brassinosteroids bind directly to the 94 amino acids comprising ID-LRR22 in the extracellular domain of BRI1, and define a new binding domain for steroid hormones.