To test the concept of self-optimization of own binding site by a metal ion, host molecules for Ni(II) ion were built on poly(ethylenimine) (PEI) by using the ethylenediamine portions of PEI and 2-carboxypyrazinyl (CP) group. Two derivatives of PEI containing CP were prepared : one by random acylation of PEI with pyrazine-2,5-dicarboxylic acid mono-(2,5-dioxo-pyrrolidin-1-yl) ester (PC-DP), and the other by acylation of PEI with PC-DP in the presence of Ni(II) ion. Between these two CP derivatives of PEI, Ni(II) binding ability was more than 10(3) times greater for the latter. Optimization by Ni(II) ion of its own binding site built on the polymer was attributed to the preassemblage of PC-DP and PEI with Ni(II) ion and the subsequent attack. at PC-DP by an amino group of PEI located in an optimal position.