The membrane-bound ribosome protein (MBRP)-complex of Staphylococcus aureus was studied using antibodies to its individual components. The four polypeptides of the complex were firmly held together, and none were present in large excess. The membrane-bound fraction of the MBRP-complex was accessible to trypsin only after removal of the membrane-bound ribosomes; it also remained associated with the membrane-bound ribosomes even after solubilization of the membranes with Triton X-100. Furthermore, the amount of MBRP-complex in the membrane was proportional to the rate of exoprotein synthesis. These results strongly suggest a role for the MBRP-complex in protein secretion.