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Forward osmosis (FO) has primarily been explored for applications in water desalination. While FO has also shown potential in concentrating dairy products, little to no attention has been paid to its potential in concentrating biotherapeutics, particularly to the very high concentrations needed for many monoclonal antibody products that are delivered by subcutaneous injection. This study demonstrates the feasibility of using FO as an alternative to ultrafiltration (UF) to achieve highly concentrated protein formulations using human Immunoglobin G (hIgG) as a model protein. The permeate flux in FO, using 1 M NaCl as the draw solution, decreased with increasing hIgG concentration due primarily to concentration polarization effects that are strongly influenced by the increase in feed viscosity for the concentrated hIgG solution. The importance of the hIgG viscosity on the FO performance was demonstrated by performing experiments with concentrated polyethylene glycol solutions and through mathematical modeling that accounts for the effects of both external and internal concentration polarization on FO performance. Batch concentration experiments with FO achieved final hIgG concentrations greater than 290 g/L compared to a maximum achievable concentration in UF of approximately 150 g/L. These results clearly demonstrate the potential of using FO, with high osmotic pressure draw solutions, to achieve highly concentrated formulations of therapeutic proteins that are beyond the capability of current UF processes.