Two portable water filtration devices were evaluated for their effectiveness in removing potentially pathogenicGiardiacysts using a rigorous testing schedule that simulated in-field use. The units were challenged daily with untreated surface water containing known concentrations ofGiardiacysts and two motile, non-pathogenic protozoans according to a defined protocol, which included both use and non-use periods. Detection and quantitation of any challenge organisms were achieved by concentrating the filtration unit effluents by membrane filtration, backflushing the polycarbonate membranes, centrifuging the resulting backflush, and examining the concentrate microscopically. At no time during the 26-day testing period were anyGiardiacysts or other challenge protozoans isolated from the filtration unit effluents. Therefore, the two portable filtration devices tested are efficient in removing high concentrations ofGiardiacysts and other protozoans.