Colloidal delivery systems including polymeric self-aggregates have been widely investigated to date, as these formulations may reduce unwanted side effects and may improve the therapeutic effects. Various characteristics of self-aggregates prepared from deoxycholic acid-modified chitosan were investigated by fluorescence spectroscopic and dynamic light scattering methods, and found to be controlled by the degree of substitution of hydrophobic groups, and pH and ionic strength of the medium. Chitosan self-aggregates can form complexes with plasmid DNA, and can be used to transfect COS-1 cells in vitro. Adriamycin can be also entrapped into chitosan self-aggregates and released in a sustained manner in vitro. These self-aggregates may find potential applications as a delivery vehicle of genes and anti-cancer drugs.