Mon-biological (polymeric/inorganic) colloids are widely used to simulate microorganisms, which are biological or living colloids. This is because of their similarity in size and: charge; Le. microorganisms are within the colloidal size range of 1 nm-1 gm and are generally negatively charged. The results of the presented work indicate that these two categories of colloids may behave in an opposing manner. The addition of E. coli bacteria to a virus suspension provides sites for the adsorption of viruses onto the surface of bacteria, resulting in the higher retention of pollovirus in a microfiltration process. However the addition of large latex particles to gold sol suspension reduces the retention of small gold particles by a microfiltration membrane. This Is because of interference by the latex particles, which inhibits the formation of the self-rejecting cake layer by the fine gold particles. The main reason for the different behaviour of biological and non-biological colloids is down to the nature of the two types of colloids. Biological colloids have adsorptive surfaces and produce extracellular polymers, allowing interactions between microorganisms, while non-biological colloids are hard spheres, with relatively smooth and non-adsorptive surfaces.