Clostridium botulinum vaccines were assayed for potency according to the method described in the European Pharmacopoeia. This method uses 20 white mice which are vaccinated with a dilution of vaccine in 0.9% saline. Twenty-one days later the vaccinated mice and ten unvaccinated control mice are challenged with C. botulinum toxin and observed for 7 days. In this study the challenge toxin was diluted in two separate diluents, 0.9% saline and peptone broth. More reproducible results were achieved with peptone broth as the diluent.