Cross-flow filtration of Escherichia coli strains was examined at the laboratory and pilot scales using Romicon 500,000 molecular-weight-cutoff hollow fiber membranes. Both the series resistance and macrosolute polarizaton models were employed to compare performances. Total dissolved solids content above 90 g/L and viscosity above 1.1 x 10(-3) Pa . s of cell-free culture media were found to decrease average filtration fluxes by over 60% both in the absence and presence of cells. Broth filtrations with culture media of dissolved solids levels below 80 g/L were influenced to a greater extent by harvest cell density. The colloidal nature of the complex nutrient responsible for the total solids increase affected prediction of filtration performance. Differences in strain filterability were observed with JM109 preferred over DH5 in high solids-containing media and RR1 preferred over JM109 in low dissolved solids-containing media. This research demonstrates the importance of cell strain and media selection in the performance of early downstream processing steps.