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Abstract: We studied the ultrafiltration (UF) characteristics of apolipoprotein A1 Milano (ApoA-1M), a highly self-associating protein considered to exist predominantly as oligomers in solution even at the relatively low concentration of a few grams per liter. Purified ApoA-1M could not be concentrated by UF beyond 25g/L in a low-salt buffer without a drastic drop in permeate flux and significant yield loss. The presence of 4M urea in the ApoA-1M solution was found to slightly lower the permeate flux at low protein concentrations or low transmembrane pressures but enabled the concentration step to reach much higher protein concentrations while maintaining reasonable flux. The reduced flux at low protein concentrations appears consistent with higher solution viscosity due to the presence of urea. Analysis of the flux data along with dynamic light scattering and diffusion studies suggests that the observed beneficial effect of urea at high protein concentrations likely results from the significant increase in the protein concentration threshold above which gel formation occurs at the membrane surface. [Copyright &y& Elsevier]