Arabidopsis thaliana SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE1 (SERK1) is a leucine-rich repeat receptor-like kinase (LRR-RLK) involved in the acquisition of embryogenic competence and in male sporogenesis. To determine the composition of the SERK1 signaling complex in vivo, we generated plants expressing the SERK1 protein fused to cyan fluorescent protein under SERK1 promoter control. The membrane receptor complex was immunoprecipitated from seedlings, and the coimmunoprecipitating proteins were identified using liquid chromatography/matrix-assisted laser desorption ionization-time of flight/mass spectrometry of the trypsin-released peptides. This approach identified two other LRR-RLKs, the BRASSINOSTEROID-INSENSITIVE1 (BRI1) receptor and its coreceptor, the SERK3 or BRI1-ASSOCIATED KINASE1 protein. In addition, KINASE-ASSOCIATED PROTEIN PHOSPHATASE, CDC48A, and 14-3-3nu were found. Finally, the MADS box transcription factor AGAMOUS-LIKE15 and an uncharacterized zinc finger protein, a member of the CONSTANS family, were identified as part of the SERK1 complex. Using blue native gel electrophoresis, we show that SERK1 and SERK3 are part of BRI1-containing multiple protein complexes with relative masses between 300 and 500 kD. The SERK1 mutant allele serk1-1 enhances the phenotype of the weak BRI1 allele bri1-119. Collectively, these results suggest that apart from SERK3, SERK1 is also involved in the brassinolide signaling pathway.