Artificial, scaffold-based binding proteins were developed as alternatives to antibodies for therapeutic and diagnostic use. Several surface exposed amino acid residues were chosen for randomization and a corresponding cDNA library was constructed. In this work, binding proteins against the extra-domain B (ED-B) of the oncofetal fibronectin were selected via phage display . By maturation via error-prone PCR highly specific binding proteins were generated. Genetic fusions of interleukin-2 and several binding proteins were produced. Both fusion partners retained affinity and biological activity. Binding proteins were analyzed by X-ray crystallography, individually and in complex with the target protein ED-B. Thus the binding mode and the amino acid residues involved in the interaction were identified. Despite numerous amino acid exchanges the typical ubiquitin fold was preserved.