Objective: To compare the potency of the two botulinum toxin (BT) type A drugs Botox® (Allergan, USA) and Xeomin® (Merz Pharmaceuticals, Germany), a formulation free of complexing proteins. The biological potency of BT drugs is determined in a mouse LD50 bioassay as described in the European Pharmacopeia. It has been discussed whether there are potency differences when compared in the product-specific mouse LD50 bioassay. However, both drugs show equal therapeutic potency in clinical trials. The biological potencies of 5 commercially available unexpired batches of Xeomin® and Botox® were determined using the LD50 bioassay for batch release of Xeomin® in a blinded fashion. Relative potencies were subjected to a quantal response parallel-line probit analysis. Potency quantification was performed using the Xeomin® reference standard qualified against the NIBSC standard. Mean values of repeat measurements were compared by a two-tailed t-test for independent data. The biological potencies of the Xeomin® and Botox® batches studied were within the range specified in the European Pharmacopeia. The potencies of the Xeomin® (103.0±5.7, n=5) and Botox® batches (101.7±6.2, n=5) were not statistically different (p=0.734). Conclusion: The potency of Xeomin® and Botox® is equivalent and confirms previous clinical experience. Conversion of Botox® and Xeomin® dosages can be performed in a 1:1 ratio allowing exchange of both BT drugs in a therapeutic setting.