This project develops a systematic performance testing protocol and specification for microfiltration (MF) and ultrafiltration (UF) membranes with respect to removal of viral and submicron bacterial pathogens. Both UF and MF have the capability to remove viruses (and submicron bacteria); however, the extent of removal is based on a number of factors including the membrane and the organism, as well as water quality and operational conditions. If membranes are to be employed on a more widespread basis for microbial removal, then their classification should be based on their ability to remove microorganisms, not on their nominal pore size. Rigorous microbial challenge studies at pilot scale are often prohibitively costly or considered hazardous. This report provides a peer-reviewed, standardized methodology with which to characterize membranes from a microbial perspective at bench scale, which is a benefit to both utilities and manufacturers. From a regulatory perspective, low pressure membranes are part of the microbial toolbox associated with the Long-Term 2 Enhanced Surface Water Treatment Rule. As a result of this project, bench-scale testing of Cyrptosporidum removal was included as methodology to evaluate new membrane products. It is important to note that the protocol was designed for both scientific rigor and ease of implementation.