The pH dependence of sulfate transport through the red cell membranes of ten mammalian (nucleated and non-nucleated) species was studied by measuring tracer exchange. The characteristic maximum of the sulfate transfer system, observed previously in the red cells of man and a number of other mammalian red cell species was found to be present in all species investigated. Although the volumes of the red cells of the various species differ by no more than a factor of 2 and the ratios of volume/surface even less, the rate constants at the respective maxima vary from 0.33 x 10(-2) min-1 (beef) to 15.50 x 10(-2) min-1 (eagle), i.e. about 45 fold. This suggests that either the number of anion transport molecules per cell and/or the turnover numbers of the individual transport molecules show considerable variations.