1. Antagonist G is a broad-spectrum neuropeptide growth factor antagonist that inhibits the growth of small cell lung cancer (SCLC) cells both in vitro and in vivo. 2. Antagonist G is metabolized in peripheral tissues by a chymotrypsin-like serine carboxypeptidase causing C-terminal deamidation and removal of the methionine residue. 3. The metabolites of Antagonist G retain neuropeptide antagonist properties and are thought to contribute to the parent peptide's antitumor activity. 4. Pharmacokinetic studies following systemic (IP) administration to nude mice revealed that the tissue distribution of Antagonist G is likely to be determined by vascular permeability. 5. Preclinical toxicology studies have been completed, and we have now started a phase I clinical trial.