The degradation of the cross-linked cationic poly(amino acid)-glutaraldehyde (GA) hydrogels by some proteolytic enzymes has been investigated using lysine (Lys)- and ornithine (Orn)-containing homo- and copolypeptides. From the experimental results, the following findings were obtained. The polylysine (PLL)-glutaraldehyde (GA) gels are degraded by trypsin, but not by chymotrypsin and papain. Optimal conditions for degradation of the PLL-GA gels are trypsin units over 500, pH 7-11, 0.5-2 M salts, and higher reaction temperatures. A pH region below 5 and a salt concentration over 4 M inhibit trypsin activity toward the PLL-GA gels. Copoly(Lys(1)-Tyr(1))-GA gels are degradable by trypsin, chymotrypsin, and even papain. Polyornithine (PLO) gels behave like PLL gels but exhibit no degradation by enzymes. Copoly(Lys(1)-Orn(1)) gels are degradable by trypsin, suggesting potential for a controlled biodegradation. The results might offer some clues to the understanding of the degradation of natural protein adhesives.