An IgG antibody is provided having a binding affinity for the CD3 antigen complex in which in the heavy chain has a variable region framework together with at least one CDR selected from the amino acid sequences of SEQ ID No 2, 4 and 6 and respective conservatively modified variants thereof and the light chain has a variable region framework together with at least one CDR selected from the amino acid sequences of SEQ ID No 8, 10 and 12 and respective conservatively modified variants thereof characterised in that the heavy chain variable region framework corresponds in sequence to the human type sequence and the light chain variable region framework includes one or more of the specific amino acids characteristic of the rodent type sequence. The novel antibody is capable of being expressed by mammalian cell expression systems at enhanced yields.