Process studies were carried out to investigate the feasibility of using tangential crossflow microfiltration to harvest antibiotic-producing cells such as Streptomyces rimosus from a whole fermentation broth. The microorganism was grown in a repeated batch fermentation up to the point of secondary mycelium formation in order to simulate broth age and mycelial morphology present in the terramycin production process. Cell separation was carried out in a Millipore Pellicon system equipped with a Durapore 0.45 mu m membrane cassette and operated in a batch concentration mode. Permeate fluxes for untreated fermentation broth were measured and compared with those for broth that was pretreated by acidification-a typical process step used to improve antibiotic recovery. Acidification from pH 7 to 4 prior to filtration changed the cell morphology and significantly reduced the inlet feed pressure required to pump the broth. In addition, pretreatment enabled higher crossflow rates and higher steady-state fluxes to be obtained at lower transmembrane pressures. Permeate fluxes for the pH-treated fermentation broth increased with operating temperature but were independent of further changes in pH.