The purpose of stability testing is to determine how the properties of a particular therapeutic vary with time under the influence of specific environmental factors. Information regarding the long-term stability of therapeutics can be extrapolated by performing an accelerated storage stability study. Here, we describe an accelerated storage stability study for the potential anti-anthrax therapeutic, EnvD, a poly-γ-D-glutamic acid (PDGA) depolymerase. Storage conditions were based on those recommended by the International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH). Owing to the high-molecular-weight and associated viscosity of PDGA in solution, loss of enzyme activity on storage can be determined as a reduction in the capacity of the depolymerase to reduce the viscosity of the polymer. This work supported by a Medical Research Council Capacity Building Studentship award.