Abstract: Solid bodies of polypeptides having repeating units of aspartic, glutamic, and aminoadipic esters are reacted with hydrazine or an amine having at least two NH.sub.2 groups until the ester moieties of the side chains are partly converted to the hydrazines or amides, and the side chains are partly crosslinked hydrazides other units by reaction with the hydrazine or diamine so as to make the polypeptide insoluble and infusible. The remaining ester moieties are saponified, and the resulting hydrophilic, but water-insoluble solid acts as an ampholyte in contact with water or other polar solvents. It may be used for capturing heavy metal ions from their aqueous solutions.