A novel approach to affinity columns is described that is based on the high avidity of biotinylated molecules for avidin attached to solid supports. Biocytin amide [Nepsilon-(+)-biotinyllysine amide] was coupled to the COOH-terminal carboxyl group of corticotropin(1-24) [ACTH(1-24)] to form [biocytin25]ACTH(1-25) amide. The ability of this peptide to stimulate steroidogenesis of bovine adrenocortical cells was within experimental error identical to that of ACTH(1-24). The peptide also binds to avidin and avidin-Sepharose, forming stable complexes. Thus, with biotin as the anchor, the adrenocorticotropically active segment of the ACTH molecule was attached to a solid support in a targeted manner. The general applicability of this principle for the attachment of peptides and proteins to solid supports is discussed.