Slow sand filtration is widely used in Europe and is enjoying renewed interest in the United States as a means of treating surface water of good quality. There are concerns, however, about the limited ability of slow sand filtration to remove organic precursor materials. Despite low filtration rates and long empty-bed contact times conventional slow sand filters have achieved limited removals of organic percursor material, with. (DOC) and trihalomethane formation potential reductions averaging less than 15-25 percent. Two modifications that have been explored for improving organic removals by slow sand filtration are granular media amendments and preozonation. The objective of this research was to evaluate slow sand filtration improvements in THM precursor removals using granular media amendments such as anionic exchange resin, granular activated carbon (GAC), aluminum oxide, clinoptilolite and anthracite. Results showed that filters amended with anionic resin and granular activated carbon (GAC) achieved significantly higher (>75 percent) removals of organic carbon and trihalomethane formation potential during an 89 day filter run than any other filter media combination studied. Includes 28 references, tables, figures.