The kinetic properties of anion transport in dog and cat erythrocytes were assessed and compared to those previously observed for human red blood cells. Sulfate and chloride transport were measured by determining the rate constant for efflux of labelled sulfate or chloride tracer ions under equilibrium conditions at 37 degrees-C. Calculation of the rate constants was described. The ionic content of the red blood cells was assessed in the presence and absence of nystatin, and the number of anionic transport sites was determined by a binding assay using dihydro-4,4'-diisothiocyano- 2,2'-stilbene-disulfonic-acid (H2DIDS). The respective efflux rate constants for anion transport in cat and dog red blood cells were efflux was pH dependent in both species. Chloride inhibited sulfate transport in cat and dog erythrocytes in a comparable manner to human red blood cells; and the human red blood cell anion transport inhibitor, 4-acetamido-4'-isothiocyano-stilbene-2,2'-disulfonic-acid (SITS), effectively reduced sulfate efflux in cat and dog erythrocytes. Cat and dog red blood cells also showed chloride self inhibition at high chloride concentrations similar to that observed in human erythrocytes. H2DIDS was a more potent inhibitor of sulfate efflux than SITS in cat and dog blood cells. The number of anion transport sites determined by H2DIDS assay was 1,200,000 and 810,000 for cat and dog red blood cells, respectively. The number of binding sites for cat but not dog red blood cells was comparable to estimates of the number of human erythrocyte anion transport sites. The authors conclude that either the turnover numbers for anion transport sites differ across species or functional heterogeneity of H2DIDS membrane binding components exists.