A considerable body of evidence indicates that interference with amino acid metabolism could be used as a basis for a concerted attack on tumor growth. Tumor cells are known to concentrate amino acids; the same transport mechanisms cause them to concentrate amino acid analogues as well. Furthermore, the growth of many tumors if affected by deficiences of certain amino acids. This apparent vulnerability of tumors points to a new approach to in vivo testing and clinical trials of amino acid analogues for efficacy against tumor growth. This approach consists of creating a dietary deficiency of an amino acid, replacing the amino acid by a toxic analogue which would be selectively concentrated by the tumor, followed at the appropriate time by restoration of the normal dietary level of the natural amino acid to allow growth of normal tissues.