The objective of this research project was to investigate the mechanism responsible for the transport of microbial contaminants, focusing on viruses, across the membrane and into the product water. Two reverse osmosis membranes and two nanofiltration membranes were chosen as representative membranes to be evaluated in a flat-leaf system. In addition to virus and solute testing, an attempt was made to develop a surrogate indicator to assess the capability for virus rejection in membrane systems. The surrogate indicator investigated was fluorescent dyed polystyrene microspheres. Includes 4 references, figures.