A membrane protein that is immunochemically similar to the red cell anion exchange protein, band 3, has been identified on the basolateral face of the outer medullary collecting duct (MCD) cells in rabbit kidney. In freshly prepared separated rabbit MCD cells, M.L. Zeidel, P. Silva and J.L. Seifter (J. Clin. Invest.77:1682–1688, 1986) found that C−/HCO3- exchange was inhibited by the stilbene anion exchange inhibitor, DIDS (4,4′-diisothiocyano-2,2′-disulfonic stilbene), with a K1 similar to that for the red cell. We have measured the binding affinities of a fluorescent stilbene inhibitor, DBDS (4,4′-dibenzamido-2,2′-disulfonic stilbene), to MCD cells in 28.5 mM citrate and have characterized both a high-affinity site (K1s=93±24 mM) and a lower affinity site (K2s=430±260 nM), which are closely similar to values for the red cell of 110±51 nM for the high-affinity site and 980±200 nM for the lower affinity site (A.S. Verkman, J.A. Dix & A.K. Solomon,J. Gen. Physiol.81:421–449, 1983). Whn Cl− replaces citrate in the buffer, the two sites collapse into a single one withK1s=1500±400 nM, similar to the singleK1s=1200±200 nM in the red cell (J.A. Dix, A.S. Verkman & A.K. Solomon,J. Membrane Biol.89:211–223, 1986). The kinetics of DBDS binding to MCD cells at 0.25 μM are characterized by a fast process, τ=0.14±0.03 sec, similar to τ=0.12±0.03 sec in the red cell. These similarities show that the physical chemical characteristics of stilbene inhibitor binding to MCD cell ‘band 3’ closely resemble those for red cell band 3, which suggests that the molecular structure is highly conserved.