In vitro studies indicate that D-penicillamine can affect the depolymerization of hyaluronate in aqueous media (induced primarily by the hydroxyl radical) in several ways. These include: Protecting the hyaluronate by quenching oxygen-derived free radicals (ODFR). Hydrated cupric ions alone and copper(II)-penicillamine complexes were equipotent in quenching ODFR but more potent than D-penicillamine alone. Initiating hyaluronate degradation by (a) directly reducing inert Fe(III) to reactive Fe(II) (which may autoxidize to produce hydroxyl radicals), a property shared with other thiols, and (b) forming reactive iron complexes that catalyse the formation of hydroxyl radicals from superoxide and hydrogen peroxide.