Steady state velocities for the rate of exchange of extracellular dithionite for intracellular sulfate through the red cell inorganic anion transport protein, known as band 3, were measured at increasing trans-sulfate concentrations at constant cis-sulfate concentration in order to directly discriminate between single and double displacement classes of mechanism. A 7-fold increase in trans-sulfate concentration resulted in intersecting Lineweaver-Burk plots under iso-osmotic conditions. This result argues against a double displacement or ping-pong mechanism. The specificity of the trans-anion effect for band 3 was illustrated by studying reversible 4-acetamido-4'-isothiocyano-2,2'-stilbene disulfonate inhibition of anion exchange. The increase in trans-sulfate increased 4-acetamido-4'-isothiocyano-2,2'-stilbene disulfonate affinity at the outer surface by about 4-fold under conditions where outside anion concentration was constant. A possible two-site ordered sequential mechanism for erythrocyte inorganic anion exchange is briefly discussed.