This chapter presents the technical considerations that have generated to date relative to constructing complementary peptides that will bind a given sequence. The studies suggest the general applicability of this technique and some potential applications of the methodology. There are many potential situations in which complementary peptides can be applied, and future studies will determine the uses. Initial studies suggest that the complementary peptide to Adrenocorticotropic hormone (ACTH), when injected in vivo, can partially inhibit stress-induced steroid production by binding ACTH. It may also be possible to use such binding peptides as substitutes for antibodies in radioimmunoassays or enzyme-linked immunosorbent assays. Along a more theoretical vein, it may be possible to identify peptide equivalents of nonpeptide ligands. If binding sites for these nonpeptide ligands could be identified from receptors or specific antibodies, then sequences complementary to these binding sites may represent peptide analogs similar to the morphine-like peptide. © 1989 Elsevier Inc.