The avian erythrocyte anion exchange protein (band 3), after labeling with [3H2]4,4'-diisothiocyanodihydrostilbene-2, 2'-disulfonic acid appears as a doublet of polypeptide chains with apparent Mr = 105,000 and 100,000 by sodium dodecyl sulfate gel electrophoresis. The structures of the two species are almost identical as determined by partial proteolysis. The copy number of band 3 molecules per chicken erythrocyte was determined to be 800,000 by quantitating the amount of [3H2]4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonic acid covalently bound to the cell surface. A comparison of human and chicken band 3 has revealed differences in their structure. Chicken band 3 differs from the human polypeptide in isoelectric point and proteolytic patterns. Antisera raised against human and chicken band 3 do not cross-react, implying that the two sera do not recognize any common antigenic determinants. There is a 6.5-fold lower activity per cell in the rate of phosphate exchange in the chicken erythrocyte which can be entirely explained by the 1.5-fold decrease in copy number per cell and the increased size of the chicken erythrocyte. This would suggest that there is no difference in the enzyme turnover number between chicken and human band 3. A major functional difference resulting from the structural differences is the inability to bind glyceraldehyde-3-phosphate dehydrogenase, a function associated with the NH2 terminus of human band 3.