Mobile colloids in soils and their underlying strata may play an important role in the translocation of some contaminants from surface sources to groundwater. This study was conducted to evaluate the role of adsorbed natural organic matter (NOM) in the transport of submicron soil colloids through a commonly occurring type of saprolite in North Carolina. Intact saprolite columns from 4 m below the soil surface were used to study the movement of a conservative tracer (3H2O) and of soil colloids with and without adsorbed NOM. For natural (i.e., untreated) soil colloids having high colloidal stability due to adsorbed NOM, the filter efficiency of the saprolike decreased rapidly to zero as increasing amounts of colloids were deposited on the pore walls in the saprolite (blocking effect). Colloid breakthrough curves exhibited little tailing, indicating that colloid deposition was largely irreversible. The colloids were excluded from about 33% of the water-filled pore space, resulting in faster transport of colloids as compared to 3H2O. When the colloids were treated with NaOCl to remove adsorbed NOM, colloidal stability and mobility were strongly decreased. For these suspensions the filter efficiency of the columns increased as increasing amounts of colloids were deposited in the saprolite (filter ripening). After addition of small amounts of humic acid (1 mg L-1) to the NaOCl-treated colloids, they exhibited very similar transport behavior as the untreated soil colloids. Stabilization of colloids by NOM and the possible occurrence of the blocking effect or filter ripening must be considered in future models of subsurface colloid transport. ¿ American Geophysical Union 1995