The binding properties of two ligands, -25,26,27,28-tetrakis(carboxyme
thoxy)calix[4]arene 1 and 38,39,40,41,42-hexakiscarboxymethoxy)calix[6
]arene 2, which show high selectivity for Ac-225(3+) (an alpha-emitter
with t(1/2) = 10 days) over Na+, K+, Mg2+, Ca2+ and Zn2+ are describe
d.