Abstract: The invention relates to a single-chain antibody which is characterized in that (a) it contains a variable domain (scFv) binding specifically to human CD3, and (b) its constant domains are derived from a human IgM molecule and comprise the C&mgr;3 domain and the C&mgr;4 domain but not the C&mgr;1 domain and the C&mgr;2 domain. In a preferred embodiment, the antibody according to the invention contains a human IgG3 hinge region between the variable and constant domains. The invention also relates to DNA sequences coding for this antibody and to expression vectors containing these DNA sequences and finally to preparations containing the above compounds, preferably for the prevention of acute rejection reactions following organ transplantation.