We have synthesized the Poly(L-histidine) (PLH) with aliphatic primary amino groups for conjugation as a new pH-sensitive polypeptide. The resulting aminated PLH suddenly precipitated out of the aqueous medium above pH 6.0 owing to the deprotonation of imidazole groups. The aminated PLH was successfully conjugated with maltopentaose by reductive amination. The resulting PLH-maltopentaose conjugate improved its water solubility at physiological pH.