The combined use of proteolytic digestion and lactoperoxidase catalyzed labelling with [125I] applied to membrane-bound or soluble pure F1-ATPase from Micrococcus lysodeikticus has allowed us to establish the topography of its alpha, beta, gamma and delta subunits within the protein molecule and with respect to the plane of the membrane. The beta subunit is most externally located to the membrane bilayer looking towards the cytoplasmic face, a position consistent with its proposed catalytic role. The alpha and gamma subunits lie in an intermediate layer between the beta subunits and the membrane, in which the gamma subunit occupies a central position within the F1-ATPase molecule in contact with the alpha subunit. The delta subunit appears to be tightly bound to the F0 component of the ATPase complex, probably buried in the membrane bilayer. A molecular arrangement of M. lysodeikticus ATPase is proposed that, taking into account the subunit stoichiometry alpha 3 beta 3 gamma 2 delta 2 (MW 420 000), accommodates the role assigned to each subunit and most, if not all, the known properties of this bacterial energy-transducing protein.