The thermostability of GARDASIL (Merck & Co., Inc, Whitehouse Station, NJ, USA), a developmental vaccine against human papillomavirus (HPV), was evaluated using an enzyme immunoassay, referred to as the in vitro relative potency (IVRP) assay and differential scanning calorimetry (DSC). Gardasil samples were stored at temperatures ranging from 4 to 42 degrees C and tested for IVRP at various time points. Extrapolation of the IVRP results indicates GARDASIL is extremely stable. The half-life of the vaccine is estimated to be 130 months or longer at temperatures up to 25 degrees C. At 37 degrees C, the half-life is predicted to be 18 months and at 42 degrees C, the half-life is predicted to be approximately three months. Differential scanning calorimetry (DSC) analysis was used to evaluate the process of protein denaturation during a rapid temperature increase (as opposed to long-term storage at a specific temperature). Differences were seen among the DSC profiles of the four HPV types tested. This indicates that small differences in the amino acid structure can have a significant effect on the intermolecular contacts that stabilize the L1 proteins and the VLP assembly. For the Gardasil samples evaluated here, DSC results demonstrated the relative overall structural stability of the VLPs, but were not predictive of the excellent long-term stability observed with the IVRP assay.