Brassinosteroids (BRs) are sterol-derived hormones that control plant growth and development. The BR receptor complex is encoded by the BRASSINOSTEROID INSENSITIVE1 (BRI1) and members of the SOMATIC EMBRYOGENESIS RECEPTOR KINASE family. For activation and deactivation, the BR receptor complex uses different posttranslational modifications and recruits various partner proteins. Here, we describe optimized immunoprecipitation protocols and variants for biochemical analyses of posttranslational modifications of BRI1 and of protein-protein interactions.