The use of antibody-enzyme conjugates directed at tumor-associated antigens to achieve site-specific activation of prodrugs to potent cytotoxic species, termed "antibody-directed enzyme prodrug therapy" (ADEPT), has attracted considerable interest since the concept was first described in 1987. Prodrug forms of both clinically used anticancer agents and novel cytotoxic compounds have been developed to take advantage of potential prodrug-generating technology employing a variety of enzymes with widely differing substrate specificities. A particular advantage of the ADEPT approach is that it may allow the use of extremely potent agents such as nitrogen mustards and palytoxin, which are too toxic to be readily used in conventional chemotherapy. Preliminary studies using an antibody-enzyme conjugate constructed with a bacterial enzyme and a murine monoclonal antibody not only have established the value of the ADEPT technique, but also have highlighted the potential problem of immunogenicity of proteins of nonhuman origin. This problem has been tackled in the first instance by the use of immunosuppressive agents, but long-term solutions are being investigated in the development of second-generation ADEPT systems, including the development of human antibody-human enzyme fusion proteins and catalytic antibodies. Such improvements, coupled with further refinement of the prodrug-drug element of the system and the wide variety of antibody-enzyme-drug combinations available, should mean that ADEPT-based approaches will form an important element of the search for the anticancer drugs of the future.