Roger Y. Tsien abstract: A new family of high-affinity buffers and optical indicators for Ca2+ is rationally designed and synthesized. The parent compound is l, 2-bis (o-aminophenoxy) ethane-A, M-A/A'-tetraacetic acid (BAPTA), a relative of the well-known chelator EGTA [ethylene glycol bis (0-aminoethyl ether)-A, A,',''-tetraacetic acid] in which methylene links between oxygen and nitrogen are replaced by benzene rings. BAPTA and its derivatives share the high (> 105) selectivityfor Ca2+ over Mg2+ of EGTA but are very much less affected by pH changes and are faster at taking up and releasing Ca2+. The affinity of the parent compound for Ca2+(dissociation constant 1.1 X 10" 7 M in 0.1 Mkc1) may be strengthened or weakened by electron-releasing or-withdrawing substituents on the