Recovery of inclusion bodies from Escherichia coli cell lysates often includes a washing step to remove associated contaminants that may otherwise interfere with refolding of the solubilized protein. The effects of denaturants typically used in the washing step on the cell lysate viscosity and membrane filtration performance were examined. Guanidine HCl alone and in combination with the nonionic detergent, Triton X-100, had the greatest effect on the cell lysate viscosity, the dead-end filtration rate and the performance of the crossflow membrane filters. Triton X-100 reduced the dead-end filtration rate, but had less of an effect than guanidine HCl on permeate flux during crossflow membrane filtration. Urea had minimal effect on either the dead-end filtration rate or the permeate flux and protein transmission during crossflow membrane filtration. The use of low concentrations of urea along with a PVDF membrane allowed for inclusion body washing without detrimental changes in the permeate flux and protein transmission during crossflow membrane filtration.