Transport of anions across the red blood cell membrane has been studied intensively in recent years under the impact of the successful identification and molecular characterization of the transport protein mediating chloride/bicarbonate exchange. The features of this electrically silent exchange process which is mediated by a 95-K Dalton intrinsic membrane protein, termed band 3 according to its position on SDS-polyacrylamide gels, have recently been treated comprehensively [67]. The transport system is characterized by a broad acceptance of substrates encompassing "regular" monoand divalent inorganic anions (halides, oxyanions) [67], but also more exotic passengers [24, 35, 74]. Numerous organic anions most likely also permeate the erythrocyte membrane in this way, e.g., dicarboxylates [25, 39, 76], aliphatic and aromatic sulfonates [17, 18, 33, 45, 64, 67, 76]; (B. Deuticke, unpublished results) as well as organic phosphates [9, 48] and even certain amino acids [108]. The question of parallel, alternative pathways of anion transport in the erythrocyte membrane has hitherto met less attention. Pathways independent of the anion exchange system and insensitive to its specific inhibitors may account for slow movements Of C1[63] and SCN[31]. Alternative routes have mainly been discussed for monocarboxylates. Being the anions of weak and often rather lipophilic acids, they have long been postulated [41, 54] to permeate by nonionic diffusion