This paper reports on a major study conducted from August 1996 through April 1997 by Irvine Ranch Water District (IRWD) evaluating the performance of an integrated membrane system with multi-objectives including organic and disinfection byproduct precursor removal, pathogen and virus removal, and biological stabilization. Prior to initiating the membrane pilot study, four different membrane integrity tests were performed. Membrane integrity tests were conducted to verify that leakage across the organic selective-nanofiltration (NF) membrane elements and the NF process components such as end seals, spacers and O-rings was not occurring. These methods included: visual inspection and probing by dye testing, EC and UV 254 rejection; vacuum testing (post pilot tested); bacteriological (E-Coli tracer study); and, particle counting. The probe test using a methyl violet dye proved unsuccessful for the NF membranes tested due to extreme fouling apparently caused by selective removal that resulted in shut down of the membrane process. Electrical conductivity (EC) and UV 254 rejection were not sensitive to predict membrane process integrity for the water tested while vacuum testing verified the failure of several elements, but is limited to pre-testing of the elements off-line and major membrane element defects. Batch particle counting results were inconclusive due to excessive sensitivity for dissolved gas microbubbles. Includes 8 references, tables, figures.