Poly(epsilon-lysine) (PL) hydrogels have been prepared by means of gamma irradiation of PL produced by Streptomyces albulus in aqueous solutions. When the dosage of gamma irradiation was 70 kGy or more and the concentration of PL in water was 1-7 wt %, transparent hydrogels (opaque hydrogels for 1-3 wt % PL concentration) could be produced. In the case of 70 kGy of gamma irradiation and 5 wt % PL concentration, the specific water content (wt of absorbed water/wt of dry hydrogel) of the PL hydrogel was approximately 160. Specific water contents of PL hydrogels decreased markedly with an increase in the dosage of gamma irradiation. The specific water contents were increased with an increase in PL concentration in the irradiated solution. This result indicates the presence of a radical scavenger in the PL solution. Swelling equilibria of PL hydrogels were measured in water or in aqueous solutions of various pHs or concentrations of NaCl, Na2SO4, and CaCl2. Under acid conditions, the PL hydrogel swelled due to the ionic repulsion of the protonated amino groups in the PL molecules. The degree of deswelling in electrolyte solution was smaller than that of other ionic hydrogels [poly(gamma-glutamic acid), poly(acrylic acid) etc.]. In addition, the enzymatic degradations of PL hydrogel were studied at 40 degrees C and pH 7.0 in an aqueous solution of the neutral protease [Protease A (Amano)] produced from Aspergillus oryzae. The rate of enzymatic degradation of the respective PL hydrogels was much faster than the rate of simple hydrolytic degradation. The rate of enzymatic degradation decreased with the increase in gamma-irradiation dose during preparation of the PL hydrogel.