Immunohistochemistry using a StAR peptide antiserum had previously revealed strong staining in rat and bovine adrenal medulla, suggesting the presence of a protein immunogenically related to StAR. Western blots of bovine medulla tissue homogenates showed the principal adrenal medullary immune-reactive species to have a higher molecular weight (50 kDa) compared to StAR protein (30 kDa). Subcellular fractionation localised the 50 kDa species principally to the medulla cytosol. StAR peptide antiserum binding to both the 30 kDa and 50 kDa species could be specifically competed by the peptide antigen. These data suggest that the adrenal medullary immune-reactive species and StAR protein are distinct entities, which share some features in common.