The hydraulic resistance and membrane fouling effects of Candida utilis in fermentation broth were investigated using Millipore PVDF 0.22-mu m membranes (GVWP and GVHP) in a stirred-cell system at 50 kPa and 700 rpm. With the various components of broth, spent medium, which contains colloidal particles and macromolecules having sizes (0.32 to 2.67 mu m) comparable with the membrane pores (actual range 0.26 to 0.63 mu m), was found to be the major contributing factor to the membrane fouling by broth through pore plugging. This led the spent medium to exhibit the highest hydraulic resistance (R(sm) of 5.8E + 12 m(-1)) and percentage flux loss (81.0%) when compared with either intact cells alone in buffer or to whole broth. Intact cells appeared to physically block and protect the pores without significant adhesion, because of the relatively hydrophilic nature of their cell walls (hydrophobicity of 5.9% at hour 36), resulting in the lowest hydraulic resistance (R(c) of 7.5E + 11 m(-1)) and percentage flux loss (19.3%). However, the hydraulic resistance and percentage flux toss of broth increased as cells aged. This was attributed to the increase in particle loading (intact cells by 15.3%, released cell contents and cell fragments) and in the hydrophobicity of cell walls. Autoclaved broth, lysed broth and aged broth, which contained a larger portion of colloidal particles and released cell contents caused a more pronounced fouling effect. This was revealed by the absence of flux recovery after depressurization with continuous stirring, even when a hydrophilic membrane was used. Furthermore, the hydrophobicity of C. utilis was found to increase with yeast extract present in medium, and use of hydrophobic membranes helped enhance the fouling effect. Overall, the degree of irreversible membrane fouling could be revealed by the value of R(sm)/R(t), and the hydraulic resistance, which resulted from concentration polarization, could be revealed by the value of R(c)/R(t), where R(t) = R(m) + R(sm) + R(c), and R(m) is the clean membrane resistance.