The fouling effects of yeast fermentation broths of Candida utilis in the presence of various commercial antifoam agents (PPG2000, B5600, and G832) up to 4.0 mL/L were studied, using Millipore polyvinylidene fluoride 0.22-mu m hydrophilic membranes (GVWP), in a stirred-cell system at 50 kPa and 700 rpm. PPG2000, which has a low value of work of adhesion (W-a of 0.81 mN/m), gave a steady flux of broth of 29 L/(h m(2)) and was found to have no significant fouling effect on the microfiltration of broth. G832, which has a high W-a, (26.0 mN/m) reduced the flux of the broth to 17 L(h m(2));i.e., by 42% when only 1.0 mL/L was used. However, B5600, which has a W-a of 14.3 mN/m, was found to enhance the flux of broth to 54 L/[h m(2)); i.e., by 86%, due to the preferential adsorption of the B5600 components onto the hydrophobic cell contents released. These results were reinforced by the depressurization experiments performed with both hydrophilic (GVWP) and hydrophobic (GVHP) membranes, using both young and aged broths. B5600 was found to be the optimum antifoam agent in this study in terms of membrane performance and defoaming efficiency.