Constrained RNA libraries of limited sequence complexity were constructed and used to select RNA molecules binding to the antigen binding site of an anti-ferritin antibody. The sequences required as primer-binding sites for the selection cycle were designed to form a predictable secondary structure, which greatly facilitated the characterisation of the secondary structures of the selected RNAs. RNA-antibody interactions were studied by real-time interaction analysis to study the dynamic aspects of binding and by circular dichroism spectroscopy to search for conformational changes upon binding. The selected RNAs were analysed with a binding site sequestering assay and were shown to compete with ferritin for binding to the antigen-binding site. The experiments described here indicate that the introduction of strong structural constraints does not have to interfere with the ability to select tightly and specifically binding RNA-molecules.