We report on an experimental study of UF membrane fouling by colloidal particles. Deposition colloidal particles during membrane filtration causes a decline in permeate flux. Membrane flux is monitored on a laboratory scale, crossflow employing UF membranes. The existing modified fouling index (MFI) uses a microfilter membrane as a quick test of feed water quality. The MFI is based on cake filtration, and thus, a model can be developed for flux decline predication. However, this MFI is not sensitive to the presence of smaller particles. Therefore, more recently MFI using ultrafiltration membranes (MFI-UF) was developed. This research investigates various critical aspects of the MFI-UF test for use as a water quality indicator; stability of the MFI-UF over time, linearity of the index with particulate concentration, and reproducibility (1) of the test (reusability of a UF module) and (2) module manufacture. Pressure dependence of the MFI-UF was also examined. The aforementioned criteria were examined using a polyacrylonitrile module with 13,000 molecular weight cutoff for low fouling (tap and process water). The MFI-UF was stable over time and directly related to colloidal concentration. The MFI-UF test was reproducible for one module with repeated testing; reproducible module manufacture was found for 80% of the test modules.