The present invention relates to recombinant binding proteins comprising a designed ankyrin repeat domain with binding specificity for CD3. In addition, the invention relates to nucleic acids encoding such binding proteins, pharmaceutical compositions comprising such binding proteins or nucleic acids, and the use of such binding proteins, nucleic acids or pharmaceutical compositions in methods of disease-localized activation of T cells, and in methods of treating diseases, such as infectious diseases or cancer, in a mammal, including a human.