The microfiltration behaviour of very dilute (ca. 10 ppm) mixtures of gold (50 nm) and latex (I mu m) colloidal suspensions has been investigated for a range of conditions (varying transmembrane pressure, and stirring) in a batch cell using hydrophobic GVHP and hydrophilic GVWP microfiltration (MF) membranes. The flux and retention for the hydrophilic GVWP were similar for stirred and unstirred conditions. For the hydrophobic GVHP stirring produced slightly lower Aux and higher retentions. The hydrophilic (GVWP) membrane showed greater flux and lower retention than the hydrophobic (GVHP) membrane in both stirred and unstirred conditions. This provides evidence of the reduced interaction between the colloids and the hydrophilised membrane. Transmission of the fine gold sol through the hydrophobic GVHP membrane was dependent on the presence of the micron-sized latex particles. In the absence of the latex the retention gold sol was essentially complete due to the rapid formation of a "self-rejecting" cake layer of colloidal gold for both stirred and unstirred conditions. In the presence of the large latex particles retention was as low as 60% (transmission 40%). The latex cake has limited ability to retain the Sold sol and also disturbs the formation of the "self-rejecting" colloid cake layer. For the sol/latex mixture chosen the "cake" resistance is determined by the resistance of the latex particles at 100 and 200 kPa applied pressure. However at 50 kPa the "cake" resistance is four times that obtained at 100 kPa. Thin-section electron micrographs show that under these conditions the fine sol particles become entrapped in the voids of the latex cake, dramatically increasing resistance.