Antimicrobial preservatives (e.g., benzyl alcohol), which are required in multidose formulations, can induce protein aggregation. In this study, the mechanism of benzyl alcohol-induced aggregation of recombinant human granulocyte colony-stimulating factor (rhGCSF) was investigated by determining the effects of temperature, pH, and sucrose on this process. rhGCSF was incubated at 25 and 37 degrees C and at pH 7.0 (phosphate-buffered saline, PBS) and pH 3.5 (HCl). Benzyl alcohol (0.9% w/v) accelerated aggregation of rhGCSF at pH 7.0, an effect that was much greater at 37 degrees C than at 25 degrees C and partially counteracted by 1.0 M sucrose. At pH 3.5, benzyl alcohol did not induce aggregation of rhGCSF. Spectroscopic studies showed that 0.9% benzyl alcohol altered the tertiary structure of rhGCSF at both pH, without detectably altering secondary structure. Structural perturbation was greater at 37 degrees C than at 25 degrees C. At both pH 7.0 and 3.5, the hydrogen-deuterium (H-D) exchange rate for rhGCSF was increased by 0.9% benzyl alcohol. Sucrose (1.0 M) partially counteracted the benzyl alcohol-induced perturbation of tertiary structure and the increase in H-D exchange rate. Thus, benzyl alcohol accelerates aggregation of rhGCSF at pH 7.0, because it favors partially unfolded aggregation-prone conformations of the protein. Sucrose partially counteracts benzyl alcohol-induced rhGCSF aggregation by shifting the molecular population away from these species and towards more compact conformations. We postulate that the absence of aggregation at pH 3.5, even with benzyl alcohol-induced structural perturbation, is due to the unfavorable energetics of intermolecular interactions (i.e., colloidal stability) between rhGCSF molecules at this pH.