Fura-2 was used to monitor RE^3^+ ion trans-membrane behavior. The characterization of RE^3^+-fura-2 interactions in solutions simulating intracellular ionic composition shows that La^3^+ and Y^3^+ form 1:1 RE^3^+-fura-2 complexes. As expected, binding of cations devoid of unpaired electrons (La^3^+ and Y^3^+) to fura-2 is associated with enhanced fluorescence, whereas for cations with unpaired f electrons (Ce^3^+, Nd^3^+, Sm^3^+, Gd^3^+, Eu^3^+, Dy^3^+, Ho^3^+, Yb^3^+) binding to fura-2 associated with decreased fluorescence. These phenomena were used to measure the intracellular free RE^3^+ ions or free RE^3^+ ions with some small molecules. The results support the assumption that RE^3^+ ions can not enter the cytoplasm of living cells and the experimental small molecules are incapable to improve the permeability of RE^3^+ ions.

Key words: CHEMICAL, LANTHANUM, YTTRIUM, MOUSE