Steroid receptors belonging to the superfamily of nuclear receptors do not exist as single monomeric proteins, but mediate their effects by interaction with numerous other proteins, e.g. cofactors for transcription, but also other proteins involved in cellular signalling . This interaction may be ligand dependent, which explains the differential effects of receptor ligands. While some receptors, e.g. the estrogen receptor, have been studied in great detail, much less is known about proteins interacting with the mineralocorticoid receptor. In this study, we aimed to identify interacting proteins using a proteomics approach involving tagged receptor constructs. After affinity isolation of MR complexes, Blue native electrophoresis revealed the presence of several populations of MR complexes differing in size and composition. During identification of interacting proteins, various heat shock proteins but also several previously undescribed potential interactors were found, including 14-3-3 . We also demonstrate here that the cytosolic mineralocorticoid receptor in the presence of detergent interacts in a ligand-selective manner with GRP78 and propionyl-CoA carboxylase beta precursor, which are found in the unliganded or aldosterone containing complex but not with spironolactone.