The removal of organic materials by biological activity is gaining greater attention as regulations become stricter and bacterial regrowth is better understood. This article compares two types of granular activated carbon (GAC) for removing the precursors of disinfection byproducts. One GAC was a coal-based carbon manufactured as an adsorbent, and the other was a wood-based carbon manufactured primarily as a substrate for biological activity. After a year-long pilot-scale evaluation, the performance of the two GACs was compared. Once the GAC systems had reached a plateau, there was little difference in the performance of the two carbon types; they removed about 15-20 percent of the precursor material. The GAC manufactured for adsorption, however, removed seven times more total organic carbon than did the other GAC. Includes 10 references, tables, figures.