The interaction between bovine serum albumin (BSA) and self-aggregated nanoparticles of cholesterol-modified O-carboxymethyl chitosan (CCMC) with different degrees of substitution (DS) of cholesterol moiety was studied by transmission electron microscopy (TEM), fluorescence quenching method and circular dichroism (CID) measurement. This interaction was started at the disaggregation of CCMC self-aggregated nanoparticles and reached equilibrium after 3-4 h. The apparent quenching constant (K-q) between BSA and CCMC self-aggregated nanoparticles calculated by the modified Stern-Volmer plot increased from 4.14 x 10(4) to 1.95 x 10(5) M-1 with DS of cholesterol moiety increasing from 3.2% to 9.8%. whereas the fraction of tryptophan residues in BSA molecule involved in the interaction decreased at the same time. Compared with free BSA, the relative alpha-helix content of BSA decreased and the unfolding of BSA by a denaturant such as urea was largely suppressed upon interaction with CCMC self-aggegated nanoparticles. DS of cholesterol moiety significantly affected the interaction between BSA and CCMC self-aggregated nanoparticles. (c) 2007 Published by Elsevier Ltd.