To gain an insight into the effect of core-crosslinking on polyion complex (PIC) micelles, the properties of PIC micelles prepared by mixing model protein, chicken egg white lysozyme with the synthesized alpha-methoxy-poly(ethylene glycol)-poly(alpha,beta-aspartic acid) block copolymer (PEG-P(Asp)-NH2) followed by glutaraldehyde addition, were investigated in detail. The added glutaraldehyde interacted with both of the lysine residues in lysozyme and P(Asp) omega-end amino groups in the micellar core, which elicited variations in the spectroscopic characteristics of the entrapped lysozyme, but no changes in its secondary structure. Micellar tolerability versus dilution, ionic strength increase, organic agent addition, and pH variation were all notably improved possibly due to strong core-shell structure-fixation resulting from this crosslinking. Such crosslinking may inhibit lysozyme denature in the presence of an organic agent, and also may prevent lysozyme escape from the micellar core usually observed due to weak association force between the lysozyme and P(Asp) segments. Additionally, the lysozyme reactivity in the micelles before and after the crosslinking seemed identical and even higher than that for free lysozyme. Such obtained stable protein-entrapped PIC micelles may find new applications in the fields of biotechnology and pharmaceutical sciences as a novel bioconjugating system. (c) 2005 Elsevier Ltd. All rights reserved.