Abstract: The invention relates to a humanized CD3 binding site, which comprises (a) a variable heavy chain domain (VH) as depicted in SEQ ID NO:8 and a variable light chain domain (VL) as depicted in SEQ ID NO:3; or (b) a variable heavy chain domain (VH) as depicted in SEQ ID NO:9 and a variable light chain domain (VL) as depicted in SEQ ID NO:4, or c) a variable heavy chain domain (VH) as depicted in SEQ ID NO:7 and a variable light chain domain (VL) as depicted in SEQ ID NO:2; or (d) a variable heavy chain domain (VH) as depicted in SEQ ID NO:6 and a variable light chain domain (VL) as depicted in SEQ ID NO:1. The CD3 binding sites have an increased stability, while the binding affinity has been retained due to mutations at positions VH111 and VL49.