The biosynthesis and chemical reactions of poly(amino acid)s produced by microorganisms are reviewed. A large amount of gamma-poly(glutamic acid) (PGA) has been produced by Bacillus strains. epsilon-Polylysine (PL) has been produced by Streptomyces albulus. As a modification of PGA and FL, pH-sensitive hydrogels have been prepared by means of gamma irradiation or the addition of a crosslinking agent to an aqueous solution of PGA and PL.