Immobilization of divalent Nickel cations provides a toot for affinity purification of proteins containing hexahistidine tags. During experiments to generate single-stranded DNA aptamers to immobilized proteins we inadvertently identified DNA sequences with affinity for Nickel-nitrilotriacetic acid (Ni2+-NTA) magnetic beads. Analysis of these aptamers revealed that affinity for the Ni2+-NTA support requires only single-stranded sequences with multiple adenosine residues. Bound nucleic acids can be eluted with imidazole. A single-stranded dA(20) affinity tag (but not other homopolymer sequences) is sufficient for immobilization of double-stranded DNA PCR products on Ni2+-NTA magnetic beads. Addition of an rA(20) sequence to an RNA transcript allowed its affinity capture on Ni2+-NTA magnetic beads, suggesting an approach for purification of poly(A) mRNA. (C) 2007 Elsevier Inc. All rights reserved.