In some patients treated with botulinum toxin (BT), antibodies are produced in association with certain treatment parameters, patient characteristics and immunological properties of the BT preparation used. Therapeutic BT preparations are comprised of botulinum neurotoxin, non-toxic proteins and excipients. Antibodies formed against botulinum neurotoxin can block BT's biological activity. The antigenicity of a BT preparation depends on the amount of botulinum neurotoxin presented to the immune system. This amount is determined by the specific biological activity, the relationship between the biological activity and the amount of botulinum neurotoxin contained in the preparation. For Botox the specific biological activity is 60 MU-EV/ng neurotoxin, for Dysport 100 MU-EV/ng neurotoxin and for Myobloc/NeuroBloc 5 MU-EV/ng neurotoxin. For Myobloc/NeuroBloc this translates into an antibody-induced therapy failure rate of 44% in patients treated for cervical dystonia, whereas for BT type A preparations this figure is approximately 5%. No obvious differences in antigenicity of BT type A preparations have been detected thus far. For the current formulation of Botox, the rate of antibody-induced therapy failure is reportedly less than 1%. To determine the antigenicity of different BT preparations in more detail, prospective studies on large series of unbiased patients with sensitive and specific BT antibody tests are necessary.