Highly purified. Control Peptide for A2990 (affinity purified antibody) and A2995-10 (antiserum) Water is a critical component of all living cells. Interestingly, tissue membranes show a great degree of water permeability. Mammalian red cells, renal proximal tubules, and descending thin limb of Henle are extraordinarily permeable to water. Water crosses hydrophobic plasma membranes either by simple diffusion or through a facilitative transport mechanism mediated by special protein ``aquaporin''. Over the last decade, genes for several members of aquaporin family have been cloned, expressed, and their distribution studied in many tissues. Aquaporin-0 or MIP26 (major intrinsic protein 26kD), and Aquaporin-1 (purified from red cells) also called CHIP-28 (channel forming integral protein, 28kD; 268aa; gene locus 7p14) has been the foundation of the growing family of aquaporins. The lens specific Aquaporin-0 represents up to 80% of total lens membrane protein. Defects in MIP26 are a cause of