Purified lac permease, the 46.5-kDa product of the lac Y gene that catalyzes lactose/H+ symport, or purified cytochrome o, a terminal oxidase of the Escherichia coli respiratory chain composed of four subunits with a composite molecular mass of 140 kDa, was reconstituted into proteoliposomes individually or in combination. The preparations were then examined by freeze-fracture electron microscopy employing conventional platinum/carbon replicas or by means of a new technique using thin tantalum replicas. In nonenergized proteoliposomes, both proteins appear to reconstitute as monomers based on (i) the variation of intramembrane particle density with protein concentration; (ii) the ratio of particles corresponding to each protein in proteoliposomes reconstituted with a known ratio of permease to oxidase; and (iii) the dimensions of the particles observed in tantalum replicas. The intramembrane particle diameters in tantalum replicas are about 20-25% smaller than those observed in conventional platinum/carbon replicas, indicating that the dimensions of the particles revealed with tantalum more accurately reflect the sizes of lac permease and cytochrome o. The diameters and heights of the permease and cytochrome o in tantalum replicas are 5.1 nm X 2.8 nm and 7.4 nm X 4.2 nm, respectively. Furthermore, a higher percentage of lac permease molecules exhibits a notch or cleft in tantalum replicas relative to platinum/carbon replicas. Importantly, the initial rate of lactose/H+ symport in proteoliposomes varies linearly with the ratio of lac permease to phospholipid, and no change is observed in either the size or distribution of lac permease molecules when the proteoliposomes are energized. The results taken as a whole provide a strong indication that both lac permease and cytochrome o reconstitute into proteoliposomes as monomers, that the permease does not dimerize in the presence of the H+ electrochemical gradient, and that both molecules are completely functional as monomers.