Crystals of subtilisin Carlsberg were insolubilized by cross-linking with glutaraldehyde in sodium sulphate solutions at pH 5.5. Depending upon the reaction time, 3 to 6 of the 9 lysyl residues were modified while none of the other amino acids appeared to be involved. The insolubilized crystals were highly active and the kinetic constants for the hydrolysis of N-trans-cinnamoylimidazole and tosylarginine methyl ester were changed only moderately, suggesting the conformations of subtilisin in the crystalline and solution states to be very similar. The activity towards casein was low, indicating that this high molecular weight substrate was unable to penetrate into the interior of the crystals. Compared with the dissolved enzyme, the cross-linked crystals autolyzed at much lower rates, had increased thermal stability, were slightly more stable in acid solutions, and had unchanged stability in alkaline solutions. Although kinetic control experiments in unbuffered solutions indicated the absence of diffusional restrictions with respect to small synthetic substrates, the diffusion of hydroxyl ions into the crystal matrix was insufficient to prevent a pH decrease within the crystals due to the protons being released by the hydrolysis of the substrates. The addition of buffers in low concentrations essentially eliminated this pH difference by accelerating the transport of protons.