Reduction-sensitive nanogels with a large swelling capacity were successfully prepared by the crosslinking reaction of biodegradable polymers in water, followed by the insolubilization of the polymers with the gradual addition of ethanol The obtained nanogels were ca 200-300 nm in diameter, and had a high water content, over 99% Moreover, these nanogels were easily decomposed by the cleavage of the disulfide crosslinkages to their thiol groups using a reductant These biocompatible and biodegradable nanogels prepared without toxic surfactants will be useful as novel earners for drug delivery systems