From a consideration of the donor preferences of toxic metals and the acceptor preferences of commonly used therapeutic chelating agents it is often possible to get a good estimate of the kinds of chelating agent which may be effective antagonists. The relative importance of extracellular vs. intracellular metal, however, introduces an important factor which can severely limit the utility of a given chelating agent with a toxic metal with which it forms very stable complexes. In general an effective antagonists of a specific toxic metal will be a chelating agent which can gain access to the metal in vivo and which forms a very stable complex, of modest toxicity with that metal ion which is rapidly excreted without damage to the kidney or liver. Such effective antagonists are not known for all metals.