Following the identification of antibodies (Abs) as agents of immunity, it was hypothesized that individuals could be both (1) protected against disease by the transfer of unmodified Ab (passive immunization), and (2) cured of established disease by Ab armed with cytotoxic agents (immunotherapy). The development of monoclonal antibody (mAb) technology in 1975 reinvigorated these ideas. Although passive immunization has been practiced with great success for many years, successful tissue targeting by systemically delivered immunotoxins in humans has been documented in only a few cases. New modes of drug delivery, engineered for mAb-based products, may enable new applications of passive immunization and may provide improved tissue targeting for immunotherapy. By allowing sustained and tissue-localized delivery of mAb-conjugates, controlled-release polymers may play an important role in this effort. This article reviews the use of mAb in treating and preventing human disease, as well as the pharmacokinetics of Ab delivery. Two areas where controlled Ab release may yield new therapies are highlighted: sustained passive immunization of the mucus secretions and immunotherapy of brain tumors.