To evaluate relationships between the extent of protein-excipient interactions, structural relaxation of an amorphous matrix, and the physico-chemical stability of a protein, human growth hormone (hGH) was lyophilized with sucrose and trehalose in a 1:2 weight ratio. The protein-excipient interactions were analyzed immediately after lyophilization with isoperibol solution calorimetry (ISC), water sorption analysis (WSA), differential scanning calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FTIR). The physical and chemical stability of hGH during storage at 50 degrees C was monitored by reverse phase (RP)-HPLC, SEC-HPLC and UV absorption spectroscopy. The hGH formulation containing sucrose demonstrated greater protein-excipient interactions and faster initial relaxation times compared to the trehalose formulation. Although both formulations had similar chemical stability (rate of deamidation), physical stabilities (e.g. degree of aggregation) were different. The hGH/sucrose formulation manifested a higher rate and lower extent of insoluble aggregate formation. The decreased amount of aggregation in the sucrose formulation could be correlated with a greater extent of protein-excipient interactions and the presence of a more homogeneous mixture. In contrast, the higher rate of aggregation in the sucrose formulation could be directly correlated with the higher molecular mobility of the matrix.